Best would be if your initial query did a limit return of 300.
For example if the users are in a user group you could use QueryGroupMembers and set maxitems to 300
It would only give you the top 300 alphabetically.
Or you could get all users (hopefully not too large a group) and use InfoTableFunctions TopN
I guess the question to start, where are the user names located, or are you trying to generate users?
