Ninad, using your tree with latest commit 4f1c34 (and I saw this behavior previously too), I have two problems.
1. Let's say I click on "B" in the user overview. Then I add a user "b6". I don't see it in the list of "B" users. Clicking on the Privileges tab shows all the users, including "b6" because on this server there are few users. Then clicking again on "B", user "b6" is not visible.
2. On another server having about 60 MySQL users, when clicking on any letter in the user overview, I see just the short "Loading" message but no list of users appears. I checked in Firebug with the Network tracing and I see in HTML the correct list of users for this letter, however it does not reach the displayed page.