Regarding User Lists and Private Chat History in MyChat

I'm looking for the following solutions:
1) What is the following setting for exactly? What does it do?
- "Make a common contact list based on the domain structure (OU)"
2) Under User List, how do I clean up that user list without deleting active users? The list is extremely long.
The original issue was that private messages aren't being saved to history. When we go back to the private user chats, they are blank. We're thinking it's possible that the many, many users on the user list might be related as though they were set as guests instead of actual users. The first question relates to this as we're trying to figure out what that option is for. What it does specifically when checked.
Any information you can provide about any of this would be more than helpful.
Thank you!
1) What is the following setting for exactly? What does it do?
- "Make a common contact list based on the domain structure (OU)"
2) Under User List, how do I clean up that user list without deleting active users? The list is extremely long.
The original issue was that private messages aren't being saved to history. When we go back to the private user chats, they are blank. We're thinking it's possible that the many, many users on the user list might be related as though they were set as guests instead of actual users. The first question relates to this as we're trying to figure out what that option is for. What it does specifically when checked.
Any information you can provide about any of this would be more than helpful.
Thank you!