Basic Math
Hints for Adding partners to a Relationship
Multiple relationships are very common in the BDSM community.  We look around us and see a lot of leather families, that seem to not only be functioning, but in fact, are thriving.  Many of us enjoy variety and want to try to create our own version of the leather family.  
In talking to people who are trying to create healthy multiple relationships, a common thread is the number of false starts.  A lot of this comes from pre-conceived notions about how relationships work and how creating a relationship fits in with a BDSM relationship.  This is particularly true in BDSM relationships with clearly defined Dominant and submissive roles and where they do not switch with each other.
In order to give a few pointers and how to add people to a relationship, I’ve come up with a few suggestions to make the process easier.  
Define what you want
No, this isn’t about whether you want tall, short, blonde, brunette or whatever.  This is about what you and the existing people in the relationship want the group dynamic to look like.  This is about structure and hierarchy.  Will you be a group marriage?  Will you be a couple who is adding a third partner as a secondary-type of relationship?  What will be the hierarchy within the group - will all members be peers or will some be subordinate to others?  How will living/sleeping arrangements be addressed?  
Be honest with your existing group
Adding members to an existing couple or group should be a group decision.  Defining changes in structure should be done with group consensus.  Although sometimes the temptation is for the primary Dominant in the group to order the submissive(s) to accept the decision and hence, whoever is added, from a practical standpoint this is a good way for the additional partner to be poorly received.  The end result is frequently infighting and destructive jealousy.
Be honest with your potential addition to the group
Anyone joining your group has the right to know what the current structure is and where they would potentially fit in.  If you do not intend for them to ever be more than a secondary relationship, they need to know that, just as much as if you hope that eventually they will move in and you will all live communally.    Obviously, things may change a long with time, but you have a responsibility to share the vision of the group with anyone who may consider joining you.
Allow all members of the group to look for additions to the relationship
In Dominant/submissive relationships in particular, it seems to be most effective to allow the submissive to pursue potential new members.  In general, it is wise to allow the partner(s) who will potentially feel the most threatened by inclusion of an additional person to do the bulk of the “shopping”.  That way they feel like they have a say and it will reduce the possibility of passive resistance if the submissive doesn’t like or is not attracted to the person the Dominant has selected.
Encourage group members to pursue independent friendships and relationships
Frequently, a secondary relationship initiated by a member of the group as an independent relationship will evolve into inclusion as part of the greater group.  Additionally, although it may go against the grain of what Dominant/submissive relationships are supposed to be about, a situation where the Dominant can have as many submissives as he or she likes, but the submissives are totally reliant on the Dominant for their emotional and sexual needs tends to create a situation where the submissives don’t get their needs met.  In the long run, this can lead to resentment and jealousy.  It is more practical to create a situation where everyone can get what they need.  Additionally, sometimes a member of the group will bring fun ideas explored in an outside relationship home for the rest of the group to enjoy.
Talk about problems
Jealousy in particular is an emotion that does not occur in a void.  Jealousy is a manifestation of other underlying feelings.  Most of the time when a partner is jealous they are actually expressing fears of abandonment or feelings of no longer being loved.  Personality conflicts are an issue also - dealing with annoyance at how someone brushes their teeth upfront is a good way of preventing that annoying from evolving into annoyance at how they brush their teeth and how they wash their hair and how they flush the toilet.
Make sure that existing partners feel included 
Sometimes a new relationship can be such that one person in the relationship may not balance the attention they focus on a new relationship well with the relationships they already have.  Existing partners should be kept informed if a member of the group is starting to explore a new relationship.  The person doing the exploring needs to be conscious of the feelings of the people already in the relationship.  The temptation to take the existing members “for granted’ can be strong in light of the headiness of emotion that frequently accompanies a new relationship.  Making sure to emphasize to existing members of the group that they are still cared for and loved and wanted will reduce a lot of the conflicts that can occur with a new relationship.
Communicate your needs
One of the common fears of people in the existing relationship is that their needs won’t get met by the other members of the group.  However, in many cases, the other members of the group, not being omniscient, aren’t really aware of what these needs are.  Stating and acknowledging needs clearly and honestly is crucial to everyone feeling able to get them met.
Take your time
Be patient and allow the relationship to evolve on its own.  Don’t push members of your group to be sexual with new members if they don’t feel so inclined.  Allow components of the relationship to progress naturally.  Don’t try to force intimacy - let it grow on its own.
Be flexible
Keep in mind that the structure and ideals that the group may have envisioned may not work in real life.  Someone that was originally brought in as partner to one member of the group may fall in love with another.  Someone who appeared initially to be a suitable addition may turn out not to be.  Someone who was an active member of the relationship may decide that the structure is not what they need at this point in their lives.  Allow the relationship to take the course that is natural for it, even if it’s not a course that was envisioned initially.  However, make sure that all members are active partners in sustaining and moving the relationship.
Have fun
If you get to a point where you spend a large amount of time managing emotional issues within the relationship, you aren’t leaving any time for fun.  Although relationships aren’t always easy, they’re not meant to be a mindless lurching from drama to drama.  If you’re working too hard at it, there’s something wrong and it’s time to sit back and figure out why the relationship isn’t fun anymore.
Multiple relationships aren’t for everyone.  However, many of the people who are in long term multiple relationships will tell you that, even though they can be more work than traditional monogamy, they find the relationships they have now more fulfilling and, in fact, cannot envision going back to a traditional monogamous relationship.  Adding new members is half the challenge - but if undertaken thoughtfully and carefully, the basic math works out to everyone’s benefit.