If you can't find a user when searching in the Users tab, you might not be using the right filters. The "Active" and "Pending invitation" status filters are always selected by default, and most search issues can be resolved by selecting all statuses.
Select all status filters
The first step to try if you can't find a user is to select all available statuses (see the image below). By selecting all the statuses, you'll enable a wider search and most likely find the user you are looking for.
If you searched using an incorrect status filter, Winningtemp can sometimes help you out by mentioning that the user can be found using another status filter. Click on the “X person if you search on all statuses” in the text below to find the user.