Whe my father died last year, I booked a skiffle band as a sort of commemoration. Ther were about 60 inmates there. Some looked as though they were on their last legs and could hardly lift a had.
We sat them all by tables ad gave them any object they could handle, such as table placemats. They bashed them against the tables for the rythmn ad had one hell of a time, and joined in with all the songs, becuase skiffle was one thing they knew.
the most wonderful thing was this one lady who didn't move at all except her lips were moving and she was singing. I talked to her afterwards and she said she really enjoyed herself.

Basically give em songs they know and really encourage them to join in in whatever way they can. Let them bash the table for the rythmn. Basically let them let their hair down and really encourage them to be a part of you. Smile and show them you are enjoying their company. Speak loud and clear and slowly for those people who do not pick words up easily.