It's all really a matter of common sense and good manners Will.

It's probably good to realise that while there are many different angles on what a 'session' is, it's not generally an occasion where you just walk in, sit down and start playing as if it's your god given right to do so. The degree to which a session values its privacy will vary from group to group and occasion to occasion but it's safe to assume it's vary rarely a true free for all where all comers are going to be welcome.