Move your body: Tap your foot; bop your head; dip your shoulders back and forth; stomp; dance... do some sort of repetitive body movement. Rhythm is a body movement thing. Bass is a body movement thing. Counting helps too, but i'm of the opinion that body movement is a more reliable tempo measure. If you're bobbing your head and shaking your shoulders back and forth, you can physically feel if you have to change direction too abruptly or too slowly. You can physically feel if you're not landing the notes on your body movements. If you're hitting the 2 but you're somewhere in between shoulder-sways, you're not hitting the 2.
You said it's bluegrass so even odds there's not a drummer or there is one. If there is a drummer and you feel you're getting lost, only play when the kick drum kicks. If there is no drummer, the bass is the drums. When there's 15, or even "only" 9 musicians playing at the same time, play less. Just hit the downbeats, and maybe fret-clack the back-beat. Send the band a big booming "THIS IS THE 1" bass note to the whole band so that everyone else knows where the rhythm is, and it's your rhythm.
The thing you can't really control is what everyone else does when there's that many players. Ideally, they'll all be following the bass (and drums if drums). And ideally the more musicians there are, the more space everyone will leave. But, musicians being musicians, there's a good chance everyone is finding all the empty spaces and filling in all the empty spaces with all the notes. But, if you're holding down the rhythm and really broadcasting the downbeats, and the other musicians lose the rhythm, that's on them. If there are enough decent players there, they'll recognize your solid rhythm through the chaos of 15 players and hopefully find a home there, and then the other players will have an easier time finding it too.
