I thnk some children are just more inclined to be passive, even if you know that it does bother them kwim? You probably wouldnt even have to bring it up with your friends as such, but if it happens again, use positive reinforcement with the children, like take the toy off the offending (for lack of a better word) child and say "how about we give this back to Nadia, you can have it when she is finished with it" and then when she is finished, make a point of giving it to the child, even if the child doesn't want it anymore (which is more than likely) it sends the message to the child that if they wait, they will still get the toy without having to take it kwim? And because of the way you do it, you can't offend their parent, unlike if you just took it off them and said no.

If it is children at playgroup, I would still do the same thing as they are children that she will be familiar with and see regularly at playgroup, so it will help her and them to learn that it isn't OK to take toys away.

It's hard when she is still little and probably can't formulate the words to ask for toys to be given back, but if she sees what you are doing, it might give her the confidence to hang onto toys or ask for them back herself eventually.