As a Beta site, Pinterest currently operates on an invite only system, so unless they have a friend on the system who can invite them, those wanna-be Pinners have to wait for the system to acknowledge their request - a process that can take some time (some are saying that they have been waiting for weeks!)
Luckily there is a faster way - of course anyone with a Facebook or Twitter account who reads this does have someone that they can ask - ME!! Just post a comment with your email address and I'll send you an invite!
The other sneaky way around the system is become a fan of Pinterest's Facebook page and post a request there! Most of the time there is a friendly Pinterest member who will be happy to send you an invite! A word of warning though - don't post your email address on the wall (sounds silly but many do!) Post your request and wait for someone to offer an invite, then inbox them your details. Still a risk, but much safer!
In closing, I'll leave you with another equally apt Pinterest definition:
(This was pinned from Tabletonic - http://tabletonic.blogspot.com/2011/09/modern-wisdom-pinterest.html)