Microsoft teams can send welcome messages when a user is added to the teams. This behavior can be altered with powershell

The current setting for the team can be checked with powershell. To run this, you need to be connected to Microsoft 365. For Microsoft 365 management you can install the “Microsoft Exchange Online Powershell Module” from Microsoft 365.

Get-UnifiedGroup -Identity 'Group Name' | fl WelcomeMessageEnabled

The outcome should be True or False. When it is set to True, welcome messages will be send. To change this setting, so welcome messages are not send, change the value to false

set-UnifiedGroup -Identity 'Group Name' -UnifiedGroupWelcomeMessageEnabled:$False