Guildhests are unlocked around level 15 and can be found on Battle Wardens located at various camps. To join a Guildhest, you have to have a full party of four players. You will ideally want a typical party setup with a tank and a healer but we were able to manage the lower level Guildhests without a dedicated healer. This may not be the case as the content scales out but the way the instances are designed, I think that party composition is not as important as in dungeons. There was little penalty for dying as you can instantly respawn and run back into the battle.
Anyway, the Guildhest itself is an instanced challenge with a time limit and objectives that vary depending on the Guildhest chosen and usually contains some type of boss mob to challenge. They are small and linear areas with a clearly defined objective.
In one example, you have to defeat a giant turtle that is constantly being healed by enemy NPCs, while Eft adds continually spawn in over time. The goal is to keep the Eft spawns at bay, eliminate all the mages, and then weaken the turtle. Once the turtle is weakened, a fire spirit spawns. You kill the fire spirit and obtain a core which is then used at a specific location to activate a type of gas that is used to trap the monster. From there, the party lures the giant turtle onto the trap and completes the Guildhest.
You get a massive amount of experience points for completing a Guildhest the first time but subsequent attempts result in a much lower amount of experience points. I am not certain if this 'first time bonus' resets daily or if you only get it once. As you complete Guildhests and level up, you unlock more. I saw at least 5 or more for each level range at 10, 15, and 25 respectively for a total of around 15-20 Guildhests. There will probably be encounters to challenge all the way up to level 50 at launch.
