Hi @frob
There are two routes to assist with this. If you add an AAD Security Group, it will only take the current members of that list and not update it going forward. You will want to either do it dynamically or manually using a Microsoft 365 Group:
1) This can be done by creating dynamic groups in AAD then create a Team from that existing group. As far as I'm aware, this is the only way to have dynamically updated groups.
2) You can also just create a Microsoft 365 group (in which you create a Team from), and manually update the users on the list. This will update the Microsoft Team's members.
Overview of dynamic membership for teams
https://learn.microsoft.com/en-us/microsoftteams/dynamic-memberships
Create a team from an existing group
If this is helpful please accept answer.