The Mound Makers are a new covenant introduced in Dark Souls III. Before you can join up with them, first you need the necessary covenant item. Do note that you have to do the following steps before beating the Curse-Rotted Greatwood boss in the Undead Settlement.
From the Undead Settlement bonfire, make your way through the area until you reach the large burning tree with the Evangelist enemy, surrounded by mobs. Instead of heading straight to progress through the level, turn right and cross the bridge. Before entering the barn, look to the right for a hidden path, but be wary of the thief enemies. You’ll soon see a ladder taking you up to the roof of the shacks. Take out the thief enemies, and then go forward to the edge of the building to see a large enemy with a cage on his back.
Simply jump down and approach him from the back, and you’ll get a button prompt to enter the cage. After the cutscene, talk to the man standing in the corner to receive the Mound Makers covenant item. Equip the item from your menu and you’ll officially be a part of the covenant.
Be sure to check out our Dark Souls III wiki for more tips and tricks.