Warstar from WarcraftPets recently posted that their guild is offering free invites to collectors for the Challenge Mode pet, Deathwatch Hatchling. The alliance guild is called Check Please and can be found on the US server Durotan-Ysera. For invites, contact Warstar (battletag can be found in the forum thread).
This is a really generous of Check Please, and I'm sure many collectors will be grateful for this offer! As a small reminder, you need to earn Revered with the guild and the pet costs 300 gold from the guild vendor. Also, if you do happen to join just for the pet, keep in mind that you're a guest in their guild. :)
I'm not sure if I'll be taking up Warstar on this kind offer; my guild will probably go for Guild Gold Challenge Modes eventually, so maybe I'll just wait for that. Then again, it's hard to resist collecting a pet that's just in reach!