Select a player randomly and put it in a role

Good evening, I would like help
In my multiplayer game I have 2 roles And I would like to be able that each beginning of game a player is randomly selected and go in the role 1 and everything else goes into the role2
I tested several methods, but it happens that several players go in the role1
And I want him to be a single player in the role1
Sorry for my English
thank you in advance