You can manage your assassins by tapping the Nomad Brotherhood icon on the bottom left. From there, you can recruit new assassins and manage the ones you already have. New Assassins are recruited by spending in-app currency. Assassins will level up when they gain enough experience. This is indicated by a gold icon appears. When leveling up, you can increase one of their stats by one. You will start with only four slots in the Nomad Brotherhood. However, this is increased by synchronizing additional Viewpoints in the main-game, which lets you to recruit Assassins from additional regions.