If you want one, leave a comment including the email address you want the invite sent to.
All comments are screened on this post, so no worries about others seeing your email address.
I speculate this is the last round of mass invites, before Gmail goes public.
So this may be your last chance to claim a username you like.
{Edit: Bolded the email address, I've already got 3 requests and none included their email addy}