How to lock or unlock users to enable or disable Private Chat messages

Meetings can be configured to begin with Private Chat messages disabled for users, or the Send Private chat messages setting can be manually Locked by a Moderator once the meeting begins:

Once set, users appear as Locked

While locked and with Private Chat messages disabled, users are unable to send Private Chat messages to each other, although Moderators can still send Private Chat messages to locked users.

To enable Private Chat messages for individual users, without unlocking the meeting's Send Private chat messages setting, Moderators can unlock them by clicking on their name in the Users list and selecting Unlock <user>:

Unlocked users can send Private Chat messages to locked users, however any locked user is unable to reply.  In the example below, Student 3 is unlocked and can send Private Chat messages to Student 1 and Student 2, however neither would be able to reply:  

Unlocked users can send Private Chat messages to other unlocked users.  In the example below, Student 2 can send Private Chat messages to Student 3, and vice versa.

To disable Private Chat message for individual users once more, Moderators can click on their name in the Users list and select Lock <user>

By using the Lock <user> feature on individual users, configuration from the Lock viewers settings is then applied to the selected users.  That is, if the meeting Send Private chat messages is set to Locked, then by locking a user, that user is then unable to send Private Chat messages.