I also have this problem. I created the group on Zotero.org and invited another member. I can see the group online, and the other member can see it in their desktop app, but I can't see it in my desktop app. I'm on Windows 10 Enterprise, Version 10.0.19044 Build 19044. Zotero is version 5.0.96.3. On Edit > Preferences > Sync > Choose Libraries, my group is selected with a green check box. Sync automatically is checked. I've tried unlinking the account with the option to remove local data, and then signing in again. I've tried restarting the application, and right-clicking My Library and then clicking Sync.
Have you clicked the main Sync button in the toolbar? Right-clicking → Sync on a library just syncs that one library.
If you're still having trouble, you'll need to start by upgrading to Zotero 6 from the download page and then provide a Debug ID for a sync. We can't provide support for older versions.
Upgrade Storage
If you're still having trouble, you'll need to start by upgrading to Zotero 6 from the download page and then provide a Debug ID for a sync. We can't provide support for older versions.