As mentioned in the topic, I cannot add a new collaborator to the existing repository. I tried to create a new repository, the result was the same. After typing the username or email list with users to choose does not show. I tried all of this combination on chrome, firefox, and mobile browser. All of the attempts failed. Please fix the bug.
See this post, same problem:
if you join an existing thread your chances of finding a solution are higher, since you might discuss the issue with other uses who are experiencing the same problem, and because older posts are more likely to receive staff support first.