The roast that keeps you in
A group keeps the person it can roast. Being left out of the joke is the exit.
Watch who gets the pictures
The person who is teased every night is still in the chat at midnight. The person nobody aims anything at goes quiet, then leaves, and everyone says they got busy. The roast was the membership.
That only works when the joke is specific and the sender is visible. A picture with their name on it, from a person they can answer, is affection with teeth. A joke sent around them, with nobody's name on the send, is a pile-on. They cannot throw one back at a crowd that will not sign it.
Aim at the claim
"You said you were leaving" is a roast. A joke about their body, their family, or a thing they cannot change is just a hit. The first one they can return. The second one they have to sit in.
If you would not say it in the room, with the laugh still in your voice, do not send it. The laugh is what makes the cut safe. Without the laugh, the cut is only rude.