I use a trick I picked up from watching Bill Monroe keep is band members up to time. I just walk around behind them and play my guitar (bass notes not banged-out chords) really loud right behind one of the culprit's ears. I hold the guitar up as though I were playing a lead into a vocal mic. Even if people at an informal jam are seated, I've been known to walk around behind the chair of someone who is trying to keep time but just can't seem to hear the beat. Of course you occasionally run into someone who just doesn't know how to play in time even when they know where the beat is. In that situation, it's best to move to another room if possible or else just make an excuse and go home.

I've also run into folks who just can't seem to play in tune. I've gone so far as to tune there instruments for them whill they're in the loo. Of course if it's a fiddle player and the fiddle is in tune, you just have to leave.

      - Mark