I’m a member there and never had any issue joining a few years ago. The only suggestions I can make is try a different browser. Internet Explorer is no longer recommended and Firefox has had issues. I use Apple Safari and haven’t had any issues. Are you using a cell phone with droid system or PC as sometimes there are issues associated with either.
You could also set up a new email account with another free internet service. I have Outlook Hotmail, yahoo and gmail that I use for different purposes. It’s possible your current email address is getting caught in the forum administrator spam filter. Some forums won’t accept members with a .com email.
If all else fails send me an private message and I’ll send a message to the bullpup forum administrator and see if I can help get you signed up. You could also contact the Bullpup Armory as he is a site sponsor and might be able to get you signed up.