Have you considering demoing the user with a couple of "sample" egistration tags? Like a "Try #4048858 or #832939 in the box below." They wya the user can see A. what the tag numbers look like and B. what entering a tag looks like. Also, I'm willing to bet that almost everyone who logs into your service w/ facebook doesn't want to log in any other way. For FB users you could skip the password and email and simple use FB for signin and send them messages when a tag is entered.

Very cool service btw, I was thinking of something just like this the other day. If, for some reason, you'd like to collaborate with someone on this idea I'd love to help.