Jenn - I feel ya, my childhood best friend also bailed, claimed it was too expensive since she is moving back to the region which I understood... then a month later booked a big trip elsewhere. Since I found that out, we haven't spoken... I was brokenhearted (still am). Hope that doesn't happen to you!
Those who really want to be there WILL be there... it's a real eye opener! Don't feel bad about the BM situation.. my fiance started out with 5, went down to ONE, and now has two (he asked another buddy). Not even his brother is going, citing some b.s. reason that I don't buy. Don't worry though, as long as you and your FI go, that's what's important!