Stand beside them and chip in on their conversation.That might seem rude. They haven’t invited you to join them, and anyway, what exactly are you going to say when you chip in?
Go up and introduce yourself. Isn’t that overly formal? Having introduced yourself, 
then what do you say? Will they introduce themselves to you? Wouldn’t you interrupt the conversation?
Wait for a break in the conversation, stand beside a friendly-
 l
ooking member of the 
group and say something like “
mi
nd if 
i
 join you?”This makes your intention clear and doesn’t seem rude or interrupt the conversation; group members can then choose whether to introduce themselves or not.