I have noticed recently an attempt by singers to pervert the meaning of Session to mean, "Sing around".
Surely a "session" (presuming you to mean a music session) is just a group getting together to make music whether sequentially or simultaneously or with or without vocals makes no difference, unless the original meaning of a session has been perverted by "instrument players"?
session (noun) -
1. a meeting devoted to a particular activity "a filming session"; "a gossip session"
2. a meeting for execution of a group's functions "it was the opening session of the legislature"
3. a meeting of spiritualists "the seance was held in the medium's parlor" Synonyms: seance, sitting
4. the time during which a school holds classes "they had to shorten the school term" Synonyms: school term, academic term, academic session