What's the name of this kind of act? It is commonly seen between friends. (I mean the act between two people, not necessarily 4 people like this one.)
Or can you describe it with a few words?

What's the name of this kind of act? It is commonly seen between friends. (I mean the act between two people, not necessarily 4 people like this one.)
Or can you describe it with a few words?

This is normally described rather literally as "putting an arm around someone's shoulder".
For example: Mike put his arm around Tracey's shoulder while they were watching the football game.
It would not be an "act". "Act" implies a performance of some kind, like in front of an audience. "Action" would be all right. "Gesture" would be a good choice. Though it generally implies movements during speech, it can also simply mean "posture" but carries a social connotation, too. A person can have a particular posture while alone, but a gesture would require at least one other person to observe it or to be the recipient of it for it to have meaning.
Example: He put his arm around my shoulder as a reassuring gesture.