C1 doesn't offer anything over Lightroom here. They both need to run the catalogue locally, so sharing a catalogue means the users need to co-ordinate between themselves, talk to each other, and make sure they check that the catalogue finishes uploading to Drop/G/CC after one user finishes work, and wait until it has downloaded to the local machine before it is reopened. If you don't take care, you can easily repeat or overwrite work.
So put one person in charge of the main Lightroom catalogue, and use mechanisms like Export as Catalog and the corresponding Import from Another Catalog to roundtrip work with the other users. This retains all your work including stacks, flags, history, VCs. In a similar situation, the main user can save his/her work to xmp, the others can read that work into their own catalogues, and you can repeat the process to get work back to the main user. That doesn't contain all your work like stacks, flags, history, VCs. Again, you need to talk to each other.
For some situations Lightroom CC can help. Sync the photos to lightroom.adobe.com and the other users could use a web browser to log in and change titles, captions, flags and ratings. They could do some adjustments too, though I'd suggest reviewing them later. In this case, any work is automatically synced back to the main catalogue.
The human element is the most important one here.