You get a private pastebin simply by thinking up a domain name no-one else is using, e.g.
http://mike123.pastebin.com or
http://this-is-my.pastebin.com. Pastes made into a subdomain only show up on that domain, making it easy for you to collaborate without the 'noise' of the regular service at http://pastebin.com. All you need to do is change the web address in your browser to access a private pastebin, or you can simply enter the domain you'd like below.
Please note that there is no password protection - subdomains are accessible to anyone who knows the domain name you've chosen, but we do not publish a list of domains used.
If you want your paste to be invisible to others, simply select the 'Private' option when you create a paste.