You get a private pastebin simply by thinking up a name that no-one else is using, e.g. http://rifers.org/paste/private or http://rifers.org/paste/ourgroup. Posts made into a private pastebin only show up there, making it easy for you to collaborate without the 'noise' of the public pastebin.

All you need to do is change the web address in your browser to access a private pastebin, or you can simply enter the name you'd like below.

Please note that there is no password protection: private pastebins are accessible to anyone who knows the name you've chosen, but the list of names is published nowhere.