Sometimes people just get caught up in the "normal" things they do everyday & they need a little extra reminding. Try to make an annoucement at breakfast in the dining room and then stop at the tables and try to drum up some enthusiasm a few people at a time. Tell them to remind all of their friends/neighbors that you may have missed. For the AL side of things, enlist the caregivers to coax the residents out of their rooms. We always tell them they can go back if they decide this activity is not for them -- but they usually stay until the end!
I also do this at dinnertime if I have a special entertainer coming in that night. Once again, I remind the caregivers to check with all of the AL residents to see if they would like to attend.