How do you lock other users from chatting in a group but allow them to see the chats you have with a particular person?

say for instance you want to interview someone but you want other users to see the chats but should not be able to interact?

I assume you want this in the chatroom .

How will you know which users will be not be permitted to interact in the chat ?

Are you thinking of adding an option that allows you to block the required user from the list of users in the chatroom ?
In that case , who is permitted to block the required users ?

Or will it be a group with fixed number of users decided before the discussion ?

Whatever may be the case, there will be a lot of modifications required, we can either do it as a paid customization or can help you with it .
Anyway , more details about your requirements would be helpful .