I migrated a test instance to 3.0 to see how it works out in our environment and noticed a mismatch with the inputs.
If you click on "Add user..." the popup asks for "User name", but you have to insert the user id accordingly to the list in /securityRealm or /asynchPeople
If the user id is "1" and the user name is "Carl", you have to insert "1" even if it asks for the name.
Then it displays "Carl" in the table, but "1" if you hover over it.
It took me a good minute to figure out if I did it wrong or it just displays something else.