Hi sub. No its not closed currently i have over 10 invites but you have to have them given to you by google. Give yourself time and with extended use google will give you invites. It took me a month to get mine and since i use my gmail account they probably assumed i use it as primary and just kept on giving me more when i gave mine out.
Hi Streaky. probably the fact that it has no or very little restrictions in its use. You can use it as a hard drive and use google to search within the emails without even opening them. Basically it works allot better than hotmail or yahoo.