I don't know of any stricly bluegrass sessions, but if you don't mind a mixed session, with people who'd be happy to jam along with bluegrass...Come along to KIncaid House hotel, Milton of Campsie on a Sunday evening..
This Sunday we will be running a session, but we also have a guest singer-songwriter from Nashville - called Bob Cheevers (the guest is called Bob, not the session).
There's a map and stuff on the website. Also - in our forthcoming music weekend at the hotel, we have guests The Moonshiners - a fabtastic bluegrass band...
Actually meant to type "fantastic" - but fabtastic seems better somehow...
Cheers
Steven