Are you trying to provide a way for Mudcatters to hear each other? Or for the rest of the world to hear the variety of music that Mudcatters produce? Or just a place where Mudcatters can go and know they'll meet some other Mudcatters face to face?
Allowing Guests is probably the way to go.
The first year that a significant number of (new) Mudcatters came to the FSGW Getaway, we had a "Mudcat Cafe" workshop/sing-around. Everybody was welcome whether Mudcatters or not, but somehow I didn't make that clear enough in the program and some non-Mudcat people felt definitely excluded.
Also, I was trying to give all the Mudcatters a chance to hear each other and that first year I think I asked non-Mudcatters just to listen. Not a good idea.... Now we just call it a "Musical Meet 'n' Greet," but introduce people by their Mudcat names if they have them, so they'll be able to recognize one another if they hadn't met before and so the non-Mudcat people will realize the contribution that Mudcat folks make to our get-together.
The thing is, a Mudcat Forum needs to be set up in a way that will enhance the feeling of community at your festival rather than seeming exclusive or divisive.