I read a definition of a crowd in one of the online drone law docs. It said that if a person can't move in any direction because someone else is standing there, there's a crowd. People need to be able to move if they see a drone falling on their head.
You're not allowed to fly over crowds but I think it depends how a crowd is defined.
Suggestions seem to be that they are uninvolved people and are too tightly packed to evade the drone if it crashes.
If they know the drone is there and are watching out for it and can evade it then maybe it's not a crowd.
But I wouldn't say for sure, and this is just from memory when I was looking a while back at the rules (I'm assuming your drone is under 250g).