When an employee/colleague has a new iPhone, they need to download the iOS app again. When they use the same Apple ID as on their previous iPhone, they can use these instructions to download the app again. These instructions only work when they use the same Apple ID.
When they use another Apple ID, they need to receive a new redeemable code. This is a code that every account can only use once. As an admin of the network, you are the one that needs to send the user a new redeemable code. You can do this by following these instructions.
The new code will be sent in an e-mail to the user. They can download the app with the normal steps now.