You can add a user in a Private or Premium Chatroom by adding the username into the chatroom's members list. Navigate to Dashboard → Chatrooms → #Chatroom Name → Properties and then click on the Chatrooms Members tab.
There is also a quick way to add a user to multiple Private or Premium chatrooms in one go. To do so, create a custom role first and then add this role into the Permissions of the chatrooms you want to give access.
Navigate to Dashboard → Roles & Permissions to create the New Role:
Then navigate to Dashboard → Chatrooms → #Chatroom Name → Properties → Permissions to add the Role to the chatroom.
And, finally add the user into the Role you have created. The user will be granted the Role permissions on all the chatrooms that the specific Role is assigned.