A team leader was the individual in charge of a group of individuals. Sometimes, there was also an assistant team leader.
In 1986, the crew of the HMS Bounty split up into three teams in San Francisco. James T. Kirk was the team leader coordinating the teams. (Star Trek IV: The Voyage Home)
In 2364, William T. Riker was team leader of an away team investigating Farpoint Station. (TNG: "Encounter at Farpoint")
In 2370, when most of the crewmembers of the USS Enterprise-D searching the surface of a planet for Data had returned to the ship, Riker called all team leaders remaining on the surface. Lieutenant Andrew Powell was the only respondent; he was ordered to pick up any other remaining personnel and take cover. (TNG: "Descent, Part II")
In 2372, drills were held aboard Deep Space 9 to combat a possible Changeling infiltration. After such a drill, personnel would check with their team leaders for the scheduled performance reviews. (DS9: "The Way of the Warrior")
The same year, Lenara Kahn was the team leader of a group of Trill scientists visiting DS9. (DS9: "Rejoined")
When the crew of the USS Voyager was marooned on Hanon IV in 2373, the crew was divided into teams with the senior officers acting as team leaders. (VOY: "Basics, Part I")
The Vori also used the designation team leader. Both Brone and Daryo were team leaders in different defense contingents in a simulation experienced by Chakotay in 2374. (VOY: "Nemesis")