Question how do i equip the side kicks in pillars 2?

Open up the inventory and click on any team mate's picture at the top. You can also select any team mate and open up the inventory directly. Equipping is done from the inventory screen.

i don´t mean items i mean the sidekicks like Ydwin and so on.
From what i understand you can have them in your party even though you already have 5 party member in them or am i misunderstanding something??

Ydwin is just like any other party member. You have to have them in your party before you can equip anything. To switch party members go to a tavern or go to the world map and select 'Supply' then to the sub menu that lets you swap members.