Users can be registered within the Users tab of an Activity's administration area.
You can either register specific users manually by searching for them in the people picker or automate them using a registration rule.
When registering specific users, they'll be registered to the Activity immediately and therefore you can set them a specific due date.
A registration rule is useful whenever you want to register users in bulk or you wish to automate registrations according to set criteria. Criteria can be related to when a user account is created, based on where they sit in the organisation or related to previous activities they are registered on to.
Registrations rules can be for a one off registration or on a recurring basis, like annually.
When using a registration rule, you can set due dates relative to when they are registered on to the activity and also reset progress if they have been registered on to the activity before.
Watch this video to learn more about how registrations work in the LMS.