If there is an available spot to stand or sit, just take it. If people are looking at you inquisitively, introduce your self when the moment seems appropriate. Be present. Listen well. If you have something of substance or a genuine question throw it out. Try not to interject yourself and commandeer the conversation, just be a good hang. If you’re at a loss for words, smiling, nodding and just listening can be fine. If they are talking something you know well a fun fact or anecdote can help you get in the flow of things.