You can have people create accounts under your domain, making them users of your account. To do this, they will go to your custom domain, click "Create free account," and they will be automatically added as users.
If someone already has a Symbaloo account and they want to be added as a user under a PRO account, they will need to email firstname.lastname@example.org with their email and the custom domain URL in order to be added as a user under the PRO account.
Do the users on my account have admin rights?
No, there is only one admin for each PRO account. Users under your account have no access to your dashboard.
Please watch this video tutorial for more information on users: