Don't really know what's that SMS.ac is about, i did tried register before as many of my friends has send an invitation to me. But nowadays i recieve all those invitation from people..zebo, sms.ac, friendster, etc etc......
Just ignore the whole damn thing. I received their so-called "invitation" all the time. Apparently, my friend told me that once you register to their website, they somehow have access to your contacts in your email and they use your name and mass-send to everybody on your contact list.
If you read the fine print on their website, they charge rates like 30 cents for incoming SMS and 50 cents for outgoing ones... Well, something like that.
Just be careful before you press the "I Agree" button.