To join a Guild, you'll need to earn at least 25 Stars in the Strategy Campaign.  When you're ready to join a Guild, tap the icon on the left side of the screen.  Go to, "Guilds" at the top, and then you'll see a list of Guilds in Twisted Towers.  "View" gives you some details about the Guild.

Press, "Join" when you find one you like!  Joining a Guild is a great way to interact with other active players and earn some extra rewards through Roll Call.