How may I simplify the registration process
You could do it using Flag and Views. 

Once you create a node flag, then all the user needs to do is click on the node flag to register to the event/interview. 

If you need to know which users have registered for an event, in Views, simply add a relationship of flag (can't remember the exact relationship name, but I am sure you can figure it out, might be "users who flagged").