I am following the tutorial https://success.outsystems.com/Support/Enterprise_Customers/Maintenance_and_Operations/How_to_Build_a_Multi-tenant_Application but i can't figure out how to get the BackOfficeUsers eSpace. 

Hi Leen,

The current forge component for the tutorial now uses the Users module, instead of having a separate user provider. But looks like the topic is not up to date to reflect that.

You should be able to use the Users module that is already present in your environment (it's a SystemComponent) instead of the BackofficeUsers mentioned there.
You will probably also have to (in Users) to give the UserManager role to the users that you want to be able to access the TenantsManagement application.

João Rosado