Another player has quit following the August 10 incident, which has caused an uproar in Norway and provoked booing from fans during a recent home game.
According to various media reports, the 12 players went out for dinner before going to a nightclub and then on to the stadium, where they were accompanied by seven women.
Security cameras reportedly captured the players having sex on the field and in the changing room.
“The club has worked intensively to get to the bottom of the very serious breaches of trust from 12 of our players,” Brann said in a statement, adding there were “many rumours and speculation” circulating.
The investigation showed “breach of guidelines, general norms, corona restriction rules and reasonable expectations of our employees,” it said.
“We have therefore decided to dismiss one of our players and given 10 other players serious written warnings,” general manager Vibeke Johannesen said in the statement.
Kristoffer Barmen, 28, who has played for the club since he was 10, was fired because of his “key role” in the night’s events, the team said.
Barmen’s lawyer Eirik Monsen said in a statement that his client was surprised by the club’s decision and felt he was being “treated unfairly”.
The player who quit was keeper Mikkel Andersen, whom Brann said was returning to Denmark after he and his family received threats at their home over the incident.
The other players involved have not come forward and have not been identified.
Police are also investigating reports of a possible sexual assault at the stadium during the incident, and one person, who has not been publicly identified, is considered a suspect.
Brann are bottom of the Norwegian Elite Division with 10 points after winning just two of their first 15 league games, but they are unbeaten in their last five games in all competitions and beat Sandeford 3-2 at home last Sunday in the league.