Say if you see a group of people that you know but aren't really friends with, talking on Facebook about doing something you like the sound of is it ever acceptable to say, "oooh can I come along?". Obviously I'd not invite myself to a party or invite only event, but if it's in a public place and they are discussing it openly on Facebook surely it's OK?
It's such a minefield. I was always taught it's rude to invite yourself somewhere, yet other people seem to do it all of the time.