Its human nature for us to not want to offend anyone we know or are associated with, but this does not mean that you must invite your associates, friends of friends, or twice removed relatives to your special event.
If confronted by a non-invitee, here are some suggested responses:
1. I really wanted to extend the invitation but space will not allow.
2. No hard feelings, I was not able to invite everyone who I wanted. I have you on the list for the next event.
3. I'm sorry you feel left out. The budget for the event will not allow me to extend the invitation.