account user

As far as I know you don't need to be invited any more. Check it out.

You can create your account without any invitation.

i think now you need not require any invitation.. directly go to the site and register for free

While Gmail was in beta they probably didn't want to overload the system. Beta is a testing period, and the number of people allowed in is often limited. Now that it's out of beta, it's open to everyone. You don't need an invitation.

I think, it is because of security purpose and to avoid making fake id's, and if a person do some illegal work through his/her id then he/she can easily be traced. One more reason can be there, just to make it popular by adding some obstacles for email ID creation. well, it is just my thoughts, i've written what i think about it, the original cause may differ from my opinion.