Folkdancing is a wonderfully social activity, which also means that it has its own peculiar set of guidelines. If you already know the dance being taught and have nothing else to do, relearn it. This gives people learning it for the first time more feet to watch.
Some songs have more than one dance, and some dances are done differently in different places.
For the sake of those learning a dance for the first time, save alternate versions for open dancing. You'll have a clearer view, and the folks who do know it will be able to dance it. Don't join at the front of a line. Holding hands in a circle while dancing is common, but no longer universal. Each session has its preference.