Microsoft Teams posts system messages to a team’s information pane to let people know about membership changes. You can’t stop Teams doing this because Microsoft doesn’t provide control over the system messages at the tenant or team level. You can obscure the names of new members by changing their display name, but maybe the best idea is not to add new members until the time is right.
https://office365itpros.com/2022/01/07/stopping-teams-posting-system-messages-about-new-members/
