Most likely you are not being able to select users as you lack the Global Permission to Browse Users.
Please look at the permission at System/Global Permissions https://<YOUR-SITE>.atlassian.net/secure/admin/GlobalPermissions!default.jspa, and you probably can see that the permission is only granted to the jira-developers group which you are not a member of.
To fix this, I would suggest for you to either add yourself to the jira-developers group or add the group which you are a member of to the permission. You would then be able to search for users.
I just tested on my current version; there is some weirdness in the UI in that the pick list doesn't expand if you don't enter anything, but typing my userid or name (and other people's) into the pick list does cause it to filter on those values... at which point, I can select them and assign them as a Component Lead. If that doesn't work, for users with the Browse and Assignable privileges, then there's probably something wrong and we'd have to go to the logs to dig further.
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