By default, candidate.fyi pulls in all of the users found in your ATS. This does not mean they immediately have access to candidate.fyi.
In order to give access to individual users:
Navigate to the Settings tab in the left navigation and then click on "Team"
Search for the user you'd like to give access to
Click on the pencil icon to the right, select a role, and toggle on "Approved to Login"
Send https://app.candidate.fyi/ to the new user and they'll be able to login
Note - by default, super users from your ATS will be automatically approved to login.