I sent a PR for this:
https://github.com/discourse/discourse/pull/4839
I replaced success page with a success alert which informs total users added to group.
I spent considerable time on this feature today trying to squeeze the information you asked for, but I realized that it’s not possible without impacting performance.
Initially I thought it will be possible because I have worked on bulk invite feature where we do provide detailed error log via PM. It’s not possible here though because instead of looping through each username/email we simply use IN condition to find out the IDs of users you are trying to add to group. So invalid username/email gets ignored, not resulting in further error. Changing this code to loop through each username/emails will result in significant performance hit when bulk adding thousands of users to a group.
I will discuss this further with @eviltrout to see if there is a possibility to add detailed error log here without significant performance hit.