Hello,
Umbrella has Allowlist-Only feature within policies. The feature will block the entire Internet, allowing only sites deliberately added to an allowlist. For example if you want to allow facebook only for 3 users then add those users and accessible domains for them in that policy.

Policies are applied to identities using a "first match" methodology based on rank (the number listed at the left of each policy), which follows a top to bottom execution order. Therefore, only the highest ranked policy that matches an identity is applied, and all subsequent lower ranking matches are ignored.
In general, the topmost policy in the list that is added to an end user applies. However, this gets more complex when a user has multiple identities such as Umbrella roaming client and an Active Directory (AD) user active at the same time.
If an identity has no matches in any custom policy, the Default Policy will apply to the identity. If you'd like to find out which policy is matched for a particular identity, check umbrella policy tester:

