If your relatives or friends are engaged to be married, their fiances (or fiancees) 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.
Beware, many unmarried people find it tremendously upsetting to not be allowed to bring a date. Prepare them for the idea and pay careful attention to where the singletons sit during dinner.
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.