By default when you create a tier, the creator enterprise has access, and if you select Allow access to this tier on the General information tab, all new enterprises will have access.
On this Enterprise access tab, you can perform these actions:
When you create or edit an enterprise, you can also change its access. To do this, go to Users → edit enterprise → Allowed datacenters → edit datacenter → Datastore tiers.
The actions on this Enterprise access tab will override the access you allow for individual enterprises. But you cannot restrict the access of enterprises that have already created disks in the tier.