What is the word for somebody who does action/is action owner

Actor is the right term, even though we more often associate it with the theater. It's all a matter of context. For example, google offers as a secondary definition the following:

a participant in an action or process.
employers are key actors within industrial relations

You might use "Executive" (the person who executes the action), or simply "Individual" to get across the notion that a person caused this action. "Person" works as well.

"Doer" is a possibility, but is rare enough in everyday usage that it might make your code harder to read.

With the additional context, the registered users are all "Registrants" or "Users" or even "RegisteredUsers" to distinguish between non-registered users if you have them.