The Basic Idea:
She had served as maid of honor at her best friend (and former on-again, off-again kinky sex buddy)'s wedding, despite the fact that she was jealous. Now, a couple of year's later, she is being brought into their marriage, as their little slave girl.
A bit more developed:
I'm looking for two female characters, both of whom would be dominant to my character, but you can work out if the relationship between the married couple is equal or if one submits to the other.
My character (MC) and one of the women (C1) were best friends, dating back to college and eventually became friends-with-privileges. Eventually, their times together came to include a kinky aspect--some lighter aspects of BDSM/D/s relationships, but at no point did this extend outside of "play." Nor was their relationship ever seen as a relationship. They both dated others, etc.
When C1 fell in love with C2, MC was completely supportive, but also a bit jealous. She'd always hoped that they'd end up together. Instead, she served as Maid of Honor at the wedding.
Now, a couple of years later, C1 and C2 have decided to bring MC--who has remained in their lives as a friend to both of them (or we could play with the idea that she faded out of their lives b/c of moving on)--into their marriage as a slave. (C1 having told C2 stories about how she used to play with MC.)