Whenever I've done Shadow Lairs, I've been in a party of four, and each member has paid his fair share. But some players don't like that system; they expect the Shadow Key owner to treat everyone else for free. I don't understand why. Just be aware that some players have weird expectations.

Shadow Keys are expensive enough, and Shadow Lairs are hard enough, that you want to go with people whom you know and trust. I mean, you want to know that they are skilled, have decent connections to the servers, have appropriate equipment for the Shadow Lair in question, have Sparks of Life to revive themselves, won't leave the party halfway through because they have to eat dinner, won't refuse to pay their share of the key cost, etc.

So try to do the Shadow Lair with friends or guildies, if you can. Otherwise, try to find one friend or guildie, and use his friends and guildies to fill out the rest of your party. As others have said, you can also ask in General Discussion, in zone chat, or just in Haven. But be careful about the people you pick up. Good luck and enjoy!