To be able to enter a stream key in OBS, you need to disconnect Twitch from OBS first. To stream to someone else's Twitch account by their stream key while being connected to the own Twitch account isn't supported by OBS.
This means your other users have to disconnect Twitch, and after that they are able to enter your stream key.
To save the connected account, they can duplicate their profile (Profile->duplicate) and disconnect Twitch in the new profile only, then enter the stream key. To switch back to their own connected Twitch account, they can simply switch back to the previous profile, where it is still connected. Rename the profiles accordingly: one for example as "My connected Twitch", and the other "Guest Twitch with key".