Well, anyone can certainly attend. The main question is whether there will be enough going on to be interesting for the attendee. There's no Filk Licensing Board you have to get certified with before you show up. :)

Drummers are often welcome in open circles, if they're good at playing along with someone spontaneously. it will vary from performer to performer. (Our Toastmaster next year, Brian Richardson, is a superb drummer himself.