Les, I find that different groups have different rules (officially acknowledged or otherwise). One song circle I attend is dominated by singers, many of whom sing or lead songs a cappella. In that group, people seem to understand that you need to be very sensitive to the nonverbal cues in deciding whether to play along or not. (If the song leader is using an instrument, some of the others often do play along.) But, with or without instruments, the goal -- though not always accomplished well -- is to follow whoever is leading the song. And if someone chooses to do a solo, a cappella or not, people honor that.

In other groups, it's the norm for songs to be accompanied by the instrumentalists and for the group to sing along, unless someone specifies otherwise.   Usually people don't ask, "Do you want us to play (sing) along?" if it's a song others know. It's not the norm in that group.

I think it's a courtesy for newcomers to get the feel of a group and its norms (or ask about them) before diving in, based on assumptions.   E.g., one jam session I attend sometimes is primarily bluegrass, fiddle tunes, and classic country/folk (e.g., Hank Williams, Woody Guthrie). Some of the regular jammers can break into a jazz version of "Humoresque" toward the end of a session with impunity, but as an occasional drop-in, I'm hesitant to venture far from the "genre" boundaries.   

Trying to "take over" someone else's song is rude in ANY setting.