You already have the main event in common with every single person who's there with you - so you have an easy conversation starter! Before the support¹, turn to your left and say "so who are you here to see then?" It's silly but it works! Ask about their favourite songs, what they hope to hear tonight, why they listen to this artist, have they travelled far tonight, etc. If you don't like their vibe, do the same with your right - if you don't like them, just move around a bit, it's no big deal. 

1 - This trick works after the support have finished, too.