Hi there! I understand you're having trouble joining or logging into the Letsfit app after reinstalling it on your Mac with OS X 10.15.7. Let's see if we can get you connected smoothly.

To start, can you tell me a bit more about what happens when you try to join? For example:
- Are you stuck on a loading screen, or do you get any error messages?
- Is this related to creating a new account, or logging into an existing one?
- Have you tried restarting your Mac after reinstalling the app?

Meanwhile, here are some helpful steps we can go through:

1. Restart your Mac: This can clear any temporary glitches from the previous app installation.

2. Check your internet connection: Ensure your Mac is connected to a stable Wi-Fi network, since the app needs internet to log in.

3. Verify app permissions: On macOS, make sure the Letsfit app has the necessary permissions to access the internet and your system.

4. Try offloading the app instead of just deleting: Sometimes, fully uninstalling isn't enough. You can try deleting the app thoroughly (drag it to Trash and empty the Trash), then reinstall from the official source.

5. Check for app updates: Make sure you're installing the latest version of the Letsfit app compatible with macOS.

6. Clear app cache or data if possible: Although on Mac this is less straightforward, sometimes deleting associated app data folders can help.

If you've already tried these or are seeing specific errors, please let me know. That way, I can help you with more detailed steps or look into account or server-side issues.

Would you like me to walk you through any of these steps?