Maybe I didn't choose the right words to express it, but I think it is important to tell them how you feel about it and that you dislike being grabbed like that.
There's a guy that became my friend's boyfriend. He came near me and put his arm around me. I didn't like it and I told him to not touch me, but he still did IT.
Yeah they might think that you are over reacting (just like my friend's boyfriend) but still THIS IS REALLY HOW YOU FEEL.
Because they are wrong to not respect your personal space. I really hate my friend's boyfriend for that. Because even after that he would come making fun of me like coming close to me , extending his arm like he wants to touch me, and he is 26 years old. Still too immature to respect people ?
However since he is dating my friend he is doing this less. But he knows I hate him,because my friend told him I hate him for touching me. He never apologized and he called me little b***h and my friend heard it she was just next to him. I didn't want to make a big fuss I was about to scream. We were at a restaurant... So, I ignored him.
Sorry, I needed to share my piece of story too.
I hope you can find a solution and that they stop.