I'm usually the one who has toys everywhere and other babies grab DS's toys.

The only time I ever say anything is when DS is ill and I point out to the mother that DS is ill, her child has DS's toy and it is a bit germ-ridden (usually at coffee mornings, not playgroups). I don't let DS put "group" toys, like the ones the playgroups provide, in his mouth: if he's biting then it's his toys ONLY. If DS is after his friend's toy then, so long as he's healthy, I'll just ask the mum if it's OK that he plays with it.