Click on "Tools" then "Options" then "Advanced."

See that bit in the middle that says "Private Network?" If you change those strings and tell your buddies those same strings (has to be exactly the same) and your IP addy... then they can connect to you and no one else can if they don't know those strings. Be sure to note what the defaults are so you can switch back to sharing with the rest of the world too.

Remember to come back... Gnutella needs ya.