In Terrain of Magical Expertise you have various party members. Each of them has its unique attacks. But two of your allies can join together and use a team attack. To do so, you will first need to get an Orb of Unity.
To get an Orb of Unity, simply progress through the game. You will receive your first Orb of Unity after completing the first regional event. You will get more Orbs of Unity later on. Team attacks use up both of the team members' turns but their power makes up for this. They do not require inputs like other attacks.
With an Orb of Unity, you can craft a team attack. You can do so at battle arenas. In the battle arena menu, go to the craft team attack option and you will see various attacks you can craft. This menu will show you which of your companions will join together for the attack as well as a description of it.
The first Orb of Unity will be used during a story mission, so you won't get to pick which team attack you want to craft. Later on, you will be able to choose which team attacks you want to craft with future Orbs of Unity. Be careful when you choose to use an Orb of Unity as they can only be used for a single combat.
For more information on Terrain of Magical Expertise, check out What are Power-Ups in Terrain of Magical Expertise and Where to find the secret path in the Luteus Labyrinth in Terrain of Magical Expertise on Pro Game Guides.