When you select everyone and put them on a team, there should be a way to send a note to the whole team: hit the three dots- message all
You have created your team in messenger. You go to send everyone a message and can't find that feature. It would be so easy if it were located in the three dots next to the team name where you can also "add a team member." A "message all" selection would streamline this process and make it more user friendly.