how do i exit a group chat on iphone without losing anything?

Worth knowing the other side of it: leaving doesn't take back anything you already sent. Everyone else keeps the whole history, your messages included. You just stop getting new ones.

Tap the group name at the top of the chat, scroll down, Leave this Conversation. The old messages stay on your phone until you delete the thread yourself. If Leave is greyed out, it's one of two things: someone in the group is on green bubbles (SMS), or there are only three of you. In that case your only option is Hide Alerts from the same screen, which at least stops the buzzing.