Answer: sitsin

AnswerCrossword Clue
SITSINAttends class without credit.
SITSINDoesn't officially enroll
SITSINAttends as a visitor (with "on")
SITSINSubstitutes (for)
SITSINAttends a meeting, with "on"
SITSINAudits, with "on"
SITSINAttends as a visitor, with "on"
SITSINSubs at bridge
SITSINProtests in a way
SITSINTakes part.
SITSINParticipates (with "on").
SITSINJoins a jam session
SITSINTakes part temporarily
SITSINSubstitutes
SITSINParticipates
SITSINObserves, with "on"
SITSINAttends, temporarily
SITSINSubs
SITSINSubs (for)
SITSINPerforms as a guest
SITSINJams with the band