1. Go to Roles & Access Control
  2. Create a custom role, choose a usertype and pages accessible to this role. Save.
  3. Go to Contacts and go to a contact's detail. The custom roles you created shall be there. Simply enable it for this contact and Save. That's it.

Please make sure you are assigning the roles to user with the associated usertype only. If you have created a custom role for administrator usertype, you shall see this custom role only for users which are registered as administrators.

So, for example, if a user's usertype is contact, only contact-usertype related roles could be assigned to that user. And if the user's usertype is administrator, only administrator-usertype related roles could be assigned to that user.