The below code should work in theory. The problem is with the way that SELECT/CASE statements work. It only seeks 1 condition to be met then it will skip to the end.
SO...if say there is a case statement ahead of 'LOGINSYSOPS' that checks for 'additionalgroup' then it will never reach this one. Does that make sense? I've always preferred just using IF statements, and avoiding CASE statements all together. But it just depends on the environment.

 Code:
Case INGROUP("LOGINSYSOPS")
    USE O: /delete /persistent
    USE N: /delete /persistent
    USE M: "\\DFS\Share"
    USE N: "\\DFS\Share"
    USE O: "\\DFS\Share"
    USE P: "\\DFS\ShareC"
    USE T: "\\DFS\Share"
    USE X: "\\DFS\Share"
    If InGroup("additionalgroup")
        use Z: "\\DFS\Share"
    EndIf