Can't manually enter username to add to group

I already have a number of people I'm following. I want to sort them into groups (great function!) However, now that the group/column is already formed, apparently the ONLY way to add someone is to find a tweet of theirs in your home feed and drag their picture over. Some folks have not tweeted in several days -- so I'm having to click through dozens and dozens of screens on my home feed, searching for their last tweet so I can drag them over and add them to that group. Is there no other way to add to a group? When you create the group, you can type in their username -- but once the group is created, I can't find that function anywhere. Am I missing something?