Hi,
First of all thank you, cause you've found a defect. . We'll report it.
Fortunately, there's a workaround.
Say, you need to add two guys: Orange and Apple
You type Orange into search -> Find him -> check him -> he appears in 'Users Selected' section
You type Apple -> Find him -> check him -> he appears in 'Users Selected' section (at this step Orange disappears from this section, but keep calm )
After it just clear "Name or username:" field and click on Search -> Users Selected field will contain both Orange & Apple
Infact, this search is currently applied to both the list of users and selected users. That's why you don't see all of them, but only currently searched