Draped in traditional white Tunisian cloaks, the women appeared Wednesday in the same court they had demonstrated in front of. Outside, dozens protested against them.
Witnesses said one European woman held a sign supporting Femen but immediately drew the ire of the crowd and was hustled away by police.
Under the Tunisian legal system, outside groups can join a trial as "injured parties", become privy to the case files and sue for damages. The addition of new plaintiffs would likely increase the duration of the trial.
The activists were calling for the release of Amina Sboui, a Tunisian member of Femen who scandalized the country by posting topless photos of herself protesting for women's rights in March. She later attempted another protest on May 19 in the religious center of Kairouan, where she was arrested.
On Wednesday she appeared before an investigating judge in Kairouan who will decide if she is to be charged with public indecency and desecrating a cemetery. She was already convicted of carrying pepper spray.
In Berlin, German government spokesman Steffen Seibert told reporters Wednesday that Chancellor Angela Merkel would likely raise questions of human rights and the rule of law during her meeting Friday with the Tunisian prime minister.
He did not say if the case of the 19-year-old German on trial would be discussed.