To resolve the issue of the Xooloo app crashing on opening across multiple iPhones, you can follow these steps:

1. Restart Your iPhones: Sometimes, a simple restart can resolve temporary glitches. Power off each iPhone, wait for about 10 seconds, and then power them back on.

2. Check for Updates: Ensure that both your iOS version and the Xooloo app are updated to the latest version. To check for iOS updates, go to Settings > General > Software Update. For the Xooloo app, visit the App Store, select your profile picture, and scroll down to see available updates.

3. Reinstall the App: Uninstall the Xooloo app from each iPhone, then reinstall it. To uninstall, press and hold the app icon until it wiggles, then tap the 'X' to delete it. Reinstall from the App Store.

4. Adjust Text Size: Some users have experienced crashes if their text size is set below 100%. Check this setting by going to Settings > Display & Brightness > Text Size and ensure it is set to normal or above.

5. Free Up Storage Space: Ensure that there is sufficient storage on each iPhone. Go to Settings > General > iPhone Storage to check available space. If it's low, consider deleting unnecessary apps or files.

6. Reset All Settings: If the problem persists, consider resetting all settings on the iPhones. This will not delete your data, but it will reset Wi-Fi passwords and other personalized settings. Go to Settings > General > Reset > Reset All Settings.

7. Consult Apple Support: If none of the above steps work, reach out to Apple Support for further assistance. They may have more detailed troubleshooting steps that can help identify the issue with the Xooloo app.

For additional insights, consider checking this support community thread that discusses apps crashing on iPhones: Apps on my iPhone crash on opening - Apple Support Community - [learn more]

If the issue continues, it may be an underlying problem within the app itself, which could require further investigation from Xooloo's development team. You can visit their official website or support page for specific updates regarding the app: Xooloo - [learn more]