Answer: sitsin

AnswerCrossword Clue
SITSIN

Joins a jam session

SITSIN

Jams with the band

SITSIN

Performs as a guest

SITSIN
Subs (for)
SITSIN
Subs
SITSINAttends, temporarily
SITSINObserves, with "on"
SITSINParticipates
SITSINSubstitutes
SITSINTakes part temporarily
SITSINAttends class without credit.
SITSINParticipates (with "on").
SITSINTakes part.
SITSINProtests in a way
SITSINSubs at bridge
SITSINAttends as a visitor, with "on"
SITSINAudits, with "on"
SITSINAttends a meeting, with "on"
SITSINSubstitutes (for)
SITSINAttends as a visitor (with "on")
SITSINDoesn't officially enroll