Guest List Advice

On Inviting Plus Ones: 

1. If your relatives or friends are engaged to be married, their fiances must be invited; their live-in romantic partners must be as well. However, if they are only dating, you need not invite their boyfriend or girlfriend. Should you decide to include some dates and not others, draw your cut-off line at a clearly identifiable place and communicate it to everyone who is not allowed to invite someone to accompany them.
2. As for your attendants -- letting them bring an escort would be a considerate gesture. It's not required, but they've done a lot for you.
3. It is impolite of a guest to ask if he or she can bring a date—but it is not impolite of you to refuse. Say, “I’m sorry, Stan, but we have very limited seating at the reception and we just can’t accommodate any additional guests.” However, if you discover that they are engaged or living together, invite your friend’s partner, either verbally or by invitation.

Martha Stewart Weddings