How can members to sign-up to a "member group"?

Hi everyone,

How can members sign-up to a specific "member group" (free of charge)?
I don't want to manually assign member groups to individual users.

Use case scenario:
I have a Tag called "cars" but I only want members who are interested in this topic to view it by signing up for the member group?

How can I achieve this?
Is there and extension for this?

Thank you SychO. I will review them both. Awesome extensions. 🙂