also re-join, 1520s, "unite again, unite after separation" (transitive), from re- "again" + join (v.). Meaning "join the company of again" is from 1610s. Related: Rejoined; rejoining.
"to answer," mid-15c., legal term, from Middle French rejoin-, stem of rejoindre "to answer to a legal charge," from Old French re- "back" (see re-) + joindre "to join" (see join). General (non-legal) meaning first recorded 1630s.