Hello Garrett Ewald, 

If you enable Messenger block for groups in studio ->pages -> groups -> view group page's settings and then create a group, you will see a block with messenger on group's page and it will use group's privacy. It means that if you create group only for friends, then groups page will be available only for the friends and thus messenger block also will be available only for friends who joined to the group and have access to the groups view page.

Also all participants of the group will be able to see talk from the group in their main messenger page, but only in case if they already posted a message to the talk, because only in this case messenger adds members to the participants list, otherwise all group's participants should open group's view page with messenger block to read talk's history.