Depending on your app Access Model, you may need to approve app users before they can access the app profile.
You can also block people at any stage.
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Instructions to approve People:

Hover over the name of an account, and available options will appear.

If the person can be approved, click the Approve link.

If the person can be blocked, click the Block link.

You can revoke approval by clicking the Revoke Approval link.

You can unlock an account by clicking the Unblock link.