If to delete the messages after other group members have read it, it is simple enough just follow this steps to delete a group chat on Facebook.
To delete a group conversation or a group chat
1. click messages at the left pane of your time line
2. select the conversation you want to manage
3. click actions
4. from the drop down list: “Delete conversations”
However if you want to delete the messages not just only from your end, but from other group members, they is where it becomes tricky.
There is no direct way of doing this, but there may be a run around, which many don’t or can’t do. If you are willing here is what you need to do if you want to delete Facebook group chat both from your own chat screen and other group members chat screen.