Try the marketplace app https://marketplace.mendix.com/link/component/80498 “Multi tenant administration”. Hope you can apply it.
Alternatively: Start out with adding the modules SystemManagement and Person to your application. See how these manage System.User and the Person’s roles. Add an entity ‘Team’ and folder ‘Teams’ (or a separate module if you feel up to it), having a 1-* association to Person (or System.User if you decided not to import the SysMan&Person modules), Generate overviewpages from the domainmodel and the logic.
I think I found an alternative way via Administration module.
The Account entity there can be linked to my apps tenant/group entity so that all users belong to one group/tenant.
Then linking all my entities to that group/tenant, allows me to restrict users to see only object that are belong to same group/tenant, the belongs to.