Allowing unregistered users to join a membership waitlist

We have set up a membership waitlist as we are at capacity.  We want to allow interested folks to join the waitlist as some members leave every month.  Our website shows our membership types to interested non-members.   If they click to register it and aren't logged in to an active account they get a message that no spaces are available and NOT given the opportunity to join the waitlist - we could require them to create an account after expressing interest in joining the waitlist if that showed as an option.  We need non - member vistiors to tell us that they want to be on the waitlist and be able to put themselves on it without expecting them to figure out they need to creeate and account and be logged in.