I beat the Minuteman main faction quests. Now I want to try the BOS faction, I can't find a way to join I try talk to Danse all he does his congratulate me on C.I.T.

You've done the main storyline with the minutemen, or the minutemen faction quests? For me doing the faction quests didn't bother anything, but completing the game's main story with the minutemen made it so I couldn't join others for some reason. Doing the game main story with that other faction, that's not BoS or Minutemen made the BoS hostile to me.

If you're not talking about the game's main storyline, have you listened to the radio transmission near the police station or investigated the airship? Both of those things will lead you into a quest to help Danse that leads to your joining.

nevermind

I quite like the fact that committing yourself to one faction eventually locks off the others. Makes for good replay potential, and actually makes sense. it also gives weight to your decisions, unlike certain other BGS games where your loyalties are never questioned and you can lead everyone.