One of my office co-workers is getting married next month. She only sent invitations to a few of the staff. (Small office of only 1

She doesn't want to offend everyone, but didn't want everyone to feel obligated also. I told her to post an invitation on the bulletin board for all to see. That would cover her bases on the offense part and then cover her on who might want to come and/or send a gift.
Are there any rules, ettiquette books, or opinions out there? Maybe Joan Lloyd could do an article on the invitations at work thing.