After you update the app, you may lose your friend list and when you try to add your friend again, you may not be able to find your friend using their Memobird ID/phone number. This is happening because you and your friend are using different Memobird app versions. Different app versions have a separate user database.
To solve this issue, you and your friend need to use the same Memobird app version. For instance, if you're using "Memobird" app that is the North American app version, your friend also needs to use the same app to create their account & Memobird ID. If they use "咕咕机" app that is the Chinese app version, the same issue will occur.