Open your report as you've been doing it. Try to set the filters so that you get at least some results, but don't worry for the moment that iut's not exactly the filter you want.
Format the report the way you want to see it (set grouping, show/hide columns, etc...)
MEMORIZE the report - give it a name you will recognize - for example, I'l use "My Sales Rep Report"
Close SO Manager
go to Windows Explorer, and browse to ..\Program Files\Microsoft Retail Managemnet System\Store Operations\Reports
There will be a file named (Using my example from above) "Memorized - My Sales Rep Report.qrp". Open this file in Notepad
Find the line that says SelCriteria = ""
To 'lock' the SalesRep part of your filter, change this to: SelCriteria = "(SalesRep.Name = 'Rep A' OR SalesRep.Name = 'Rep B' OR SalesRep.Name = 'Rep C' )"
Save the file and reopen SO Manager.
Go to Reports/Memorized and open your memorized report Any Filters you add (Like Status = Open) will be included in the filter an an AND condition with the permanent selection Criteria above.
I wasn't sure what report you were starting from, so I didn't know the proper context for Status - it really probably makes more sense to make that the 'permanent' condition...