If I have to add more testers (especially outside clients), What is the best way to add it?
Since I don't know the outside testers' UDID, how do I tackle this situation? The only way I can see is request their UDIDs and add them to the profile, download the updated profile and rebuild it and send it to these outside testers. Seems very cumbersome.
Actually not. But, you can ask users for their UUID before your build and then add them to your developer account and then release the build. Their are few methods you can Google of how users can get their device UUID.
Or you can use Test Flight beta for external users, which requires Email ID of testers only and a beta review from Apple which takes less than 24 hours, if that's not a problem. It also makes sure your app is fit for app store at every iterating and will significantly reduce chances of rejection at final stages.