The Clanstone can be bought in the store for 20000 cubits. It is one of the only blocks in the game that is 3 blocks high. Placing down a clanstone for the first time will prompt you to name your clan, which will be forever binded to your clanstone unless changed with 2500 clan XP. You can recruit people to the clan when they are standing atop the Clanstone by clicking on invite option/button, but if multiple people are standing on the Clanstone you won't be able to invite.