WordNet (r) 3.0 (2006):

sit in
    v 1: attend as a visitor; "Can I sit in on your Intermediate
         Hittite class?"
    2: participate in an act of civil disobedience

Moby Thesaurus II by Grady Ward, 1.0:

116 Moby Thesaurus words for "sit in":
   assemblee, assembly, assignation, at home, ball, be enfranchised,
   be implicated in, bear a hand, beef, bitch, boggle, boycott, brawl,
   call in question, caucus, challenge, chip in, colloquium,
   commission, committee, complain, complaint, compunction, conclave,
   concourse, congregation, congress, contribute, conventicle,
   convention, convocation, council, cry out against, dance, date,
   demonstrate, demonstrate against, demonstration, demur, demurrer,
   diet, dispute, eisteddfod, enter a protest, enter into, exception,
   expostulate, expostulation, festivity, fete, forgathering, forum,
   gathering, get-together, go into, grievance, grievance committee,
   have suffrage, help decide, holler, housewarming, howl,
   indignation meeting, join, join in, kick, levee, make the scene,
   march, meet, meeting, nonviolent protest, object, objection, panel,
   partake, partake of, participate, participate in, party, picket,
   picketing, plenum, press objections, prom, protest,
   protest demonstration, protestation, pull an oar, qualm, quorum,
   raise a howl, rally, reception, remonstrance, remonstrate,
   remonstration, rendezvous, scruple, seance, session, shindig,
   sit in on, sitting, soiree, squawk, state a grievance, strike,
   symposium, synod, take part in, teach in, teach-in, turnout, vote,
   yell bloody murder