Bluegrass and Old time don't seem to sit well together in the same session. Old time is mostly about fiddle tunes, with maybe a few songs, with all the instruments simultaneously trying to make the tune sound better, while bluegrass is mostly about songs interspersed with instrumental solo breaks - totally different structures which don't blend. So pick the right genre for you and stick to the one.
For old time you really need a good fiddler who's heard and relates to a lot of old recordings, and has a large repertoire of tunes. You need a good guitar player who can provide a solid rhythm and hold the session together. The others - banjos, mandolins - will pick up the tunes and try and make the fiddler sound better - at least thet's what I do in a session.
Which is your city, Sean? If you're in the UK you can advertise/ promote through Foaotmad, which has a quarterly magazine and a mailing list, which you can feed by writing to publicity@foaotmad.org.uk
Good luck with it - it's not easy.
Ray