Per standard regulations concerning Tree Houses, Clubhouses, Forts, etc. the official policy is "GIRLS ARE NOT ALLOWED". However, there is plenty of precedent showing exceptions to this policy provided one, all, or a combination of the following is undertaken or provided by the applicant female.
1. Be cootie free
2. No kissing
3. Gift, or gifts generally of a sporting nature which are difficult to refuse may help gain entry
4. No dolls
5. No dumb questions
6. No crying
I believe the list is longer than this, but those are the basics.
If you wish to bypass all of this, you could have your mom talk to the WGT Nation members mom's and I'm certain you'll get an invite.
Good luck.