You can create multi-tenancy in a Mendix app by associating each entity to the ‘tenant’ entity(direct or indirect).
Furthermore, every account should be linked to a specific 'tenant’.
Then for every entity you should setup the entity acces to allow each user roles to only see and manage the entity's with it's own tenant.
There is an appstore module that could help you get started: https://appstore.home.mendix.com/link/app/80498/
Also take a look at these previous asked questions about this subject: