After the Civil War event in the Marvel Comics universe, registration is required for super-powered beings.
Is there a cap on this? Do some people get out of this by being 'useless'? Are people enlisted, as well as registered?
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Registration is no longer required! Huzzah!
However, back during the heyday of registration, Avengers: The Initiative gives us a good look into what powersets are registered, and what ends up happening.
Cloud 9 is the example that immediately comes to mind. At the beginning, she has no interest in superheroics, but the Initiative program turns her into a sniper. Butterball, on the other hand, washes out of the program since he's not very useful in combat.
To answer your questions: If there is a cap, it would only be to weed out people with purely ornamental powers. Even these people might have to register, although training seems unlikely. As shown in Avengers: The Initiative, registrees are trained to become superheroes, which serves a loose form of enlistment.