sit in on (?.)

• attend
• attend to
• auscultate
• be all ears
• be enfranchised
• be implicated in
• bear a hand
• bend an ear
• bug
• chip in
• cock the ears
• contribute
• eavesdrop
• enter into
• examine by ear
• give attention
• give audience to
• give ear
• go into
• hark
• have suffrage
• hear
• hear out
• hearken
• heed
• help decide
• intercept
• join
• join in
• lend an ear
• listen
• listen at
• listen in
• listen to
• make the scene
• partake
• partake of
• participate
• participate in
• pull an oar
• sit in
• take part in
• tap
• vote
• wiretap