I only saw tab and y in setting keybind menu, but they do nothing! How to change party?

Nevermind, I got it.

How do you do it please?

If your party is full, talk to the character, you want to dismiss for now, choose the 'I want you to stay in camp'-option and they will leave the party and go to camp. Then talk to the character, you want to have in the party and tell them, that you want them to accompany you and they join the party.