Place the Catalog file and the previews folder, in the same folder as the catalog, in a folder both users have access to. Normally LR places the catalog in the C:\User\YourUserName\Pictures folder. a Different user does not have access to to that folder unless you specifically give them access through the Security settings.
If you created a folder in the ROOT of the C drive, calling is something like Lightroom Shared Catalog, then Moved the original catalog folder under your username folder to the folder you created then when opening LR you point it to using that catalog in that newly created folder. you should also check the permissions on that new folder and grant Full Access to it from both Usernames.