Multi-Player setups can be used to implement local co-op or a variety of gameplay features. Note that for networked multiplayer games, multiple Players are not needed, as each client will be its own Player.
Since each Active Grid is only capable of working with a single Player, multi-player setups naturally require multiple Active Grids to be present in your scene.
Each Active Grid can be synced to a different World, however it's usually more common for them all to be synced to the same one.
Active Grids are certainly the easiest way to drive dynamic World updates, however they are not the only way. You are free to add as many Players as you want and track there movement yourself. Then, based on this tracking, you can load World Regions or manually add users for World Cells. The World does not care where the users of its World Cells come from, and it has been built to handle users from as many sources as you can throw at it, so there is no issue with having multiple Active Grids synced to the same World, or paring Active Grid Players with non Active Grid Players.
A Cell User is simply any source that needs a World Cell to be loaded, or more specifically, specific LOD Assets of the World Cell from a particular World Grouping to be loaded.
The World periodically checks to see if any cell users have been added or removed. Competing add/remove requests for the same LOD of the same World Cell on the same World Grouping cancel each other out. If after processing all add/remove requests any requests remain, an actual World Update is initiated.
Each World Cell keeps track of the number of users for each LOD it can load assets for. The asset chunks belonging to the highest quality LOD that has users are always loaded, since there is no disadvantage in doing so for the users that need the lower quality LOD assets. This means it does not matter if one LOD has 10 users and a higher quality LOD only has 1. The higher quality LOD assets will remain loaded until the single user has been removed (and then a transition to the lower quality LOD assets will occur).
If a World Cell has lower quality LOD Assets loaded and then gains users for higher quality LOD assets, a transition will occur from the lower quality LOD assets to the higher quality ones, and the lower quality LOD assets will be removed. Of course, if that user is then removed, new assets for the lower quality LOD will be loaded and transitioned back to. Only when all users for every LOD reaches 0 is the World Cell completely removed.