Yep Ellenpoly you've got that about right but you also need to know that when you come into a new event, you are excpected to know or find the understandings (or rules) for a particular event. The onus is on you, not the regulars to do that.

As you say, it does need raggytash to chime in but it remains unclear to me the nature of the event he was attending. I notice I'm not the only one unclear as only a few posts ago, Richard Bridge said:

But to come back to the original question, surely the root of the determination lies in the nature of the session. If it was a participative session (whether song or tune) then Raggytash by seeking to exclude participation (either by pre-announcement, or by spontaneous combustion) is at fault - but if it was a "take turns" session, then the intervener was at fault in joining in if not invited.

Another thing to bear in mind is that a person used to one event or format can easily and wrongly assume that another is the same. I'm not saying that is the case here but without that sort of knowledge, I think it rather foolish to give out the "tell the offender to shut up" type of advice.

Sometimes how to deal with it is simply "Deal with it"!

Jon