The friendship began when Izuku met Iida and Uraraka in the U.A. Entrance exam, first admiring their powers and later saving Uraraka from one of the test robots, earning respect from Iida and growing close to both of them after they became classmates. They are regarded as the "original DekuSquad" and are Izuku's closest friends.
Todoroki “joined” DekuSquad following the U.A. Sports Cavalry Battle Arc, after he got over his one-sided rivalry with Izuku and came to terms with his quirk as well as his father, which was the source of most of his antisocial behavior. In addition, when Izuku sends out his location during the fight with Stain in the V.S. Hero Killer Arc, Todoroki is the first and only one to come to his aid, stating to his father that his friend is in trouble. After this, he is shown to open up to his classmates more and is often seen eating with Izuku and his friends.
During the USJ incident, Izuku and Tsu were forced to fight off villains together, and showed commemorable teamwork in defeating them. In addition, when the Bakugou Rescue Squad recruits Deku for their mission, Tsu doubts their plan and tries to convince them not to go, stating that they would be “no different than villains”, however, they ignore this statement, and go on the rescue mission anyways. Tsu later feels bad about doubting them, which leads to both parties apologizing once the mission is over and thus repairing their friendship.
There is controversy to whether Tsuyu actually belongs in DekuSquad, but she's widely accepted to be a member as she is shown eating and hanging out with the group multiple times.
The group was also dubbed and is sometimes referred to by fans as the "Deku Squad" or "Izucrew"
In some fanworks other characters are added to this group such as; Shinsou, Aoyama and Momo. And less often Tokoyami. However, they are not actual members of the canon.
The alternative, or rival, group is "BakuSquad", which consists of Bakugou, Kirishima, Denki Kaminari, Sero, and Mina. Eh once or twice Kyouka Jirou