[cypher]

Hamsters don't get fleas, they can get mites from some things like hay & wood items, they also have mites that are naturally present all the time but only become a problem if their immune system is compromised by illness or stress so it's most likely you are seeing normal grooming or something in the cage could be irritating your hams skin.
If your hamster has been scent marking a lot as often happens in a new cage or after a cage clean then you will see more grooming than normal.
If mites are a problem irritation usually begins around ears & eyes so if you see more scratching in that area it could be that.
If they seem to be scratching/grooming more when you handle them (if that's what you mean by mingling with them) then it could just be a scent thing.