MyClub assumes that there may be some users who won't have a login. These users are Club Managed Users.
For these users you will need to make changes on their behalf. There are two ways to achieve this. Which is used is personal preference, they both result in the same outcome.
Any user with the duty admin permission can assign and unassign on behalf of any user.
You must also be a club admin to see this option. If enabled you can "switch" to temporarily become the other user via the link at the top right.
In this case the user experience is a little different, the UI behaves as if the user had signed in. You will see a Volunteer button. The Assign button is not available.