Community page

Can anyone tell me how I can setup one of those community registration pages (I saw one for Ben&Jerry's, for instance)? I would like to have one on my site, to send people to my island.
I also wonder if it is possible to directly assign people signing up in such a form, into specific groups? I am thinking about having different groups that can get access to different parts of the island.

That is accomplished with the registration API.

I don't quite understand what you mean with that. Do you have more information on how I build such a page? Perhaps somewhere I can see an example?

Yes, there are examples posted there.
https://secure-web10.secondlife.com/developers/third_party_reg/#examples
Dunno if this direct link will work because it says https, but everything is under the first link.