Whoops! I wasn't paying attention. My mistake! Ignore this comment.
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
Well would ya look at that! Excellent. Thanks very much for the info - this works!