Thank you very much. This is what it says in the invite section.
Because we're currently only offering Gmail as part of a preview release and limited test, we don't have details on when Gmail will be made more widely available.
As we make way for more accounts, we may periodically allow you to invite others to join Gmail. When we do so, you will see an invitation link in your inbox.
If you do not see the invitation link, that means that we are unable to provide you with additional invitations at this time.
To invite someone, click on the 'Invite a friend to join Gmail!' link in your inbox. You will then be asked to provide an email address and a personalized message to your invitee.
The person you invite will be sent a message that includes a link to join Gmail. That registration link will be valid for three weeks and can be used to create only one account.
Should your invitee have problems creating a Gmail account, please let us know by clicking on the 'contact us' link in the grey box below.
Please note that the link may not always be available. When you are allowed to invite others to join Gmail, the number of invitations that you can send will be indicated at the top of the page after you click on the link. Once you have used all of those invitations, we will not be able to immediately issue more invitations.