There are several so called open trackers. It means they aren't tied to any website. If you build the .torrent file indicating one of those sites, they will accept and track the torrent without question (or the need to notify/upload anything).
Then you simply send the .torrent to those you want to allow to get on board.
The down side is unlike a site like MySpleen, people won't know it exists to join in later.
The upside is that it's more exclusive.