From your home screen click on the groups section, You will then be redirected to your list of rooms you own and rooms you are in please select a room you own to put the password on and press on it to open it up.
Once open, in the top right hand corner open the edit page by clicking on the pencil icon as shown above. Find the password section and click the checkbox, then enter a password.
Please note this can only be done with a premium group and if it loses premium for any reason the password will be lost.