Calculating percentages using multiple columns

Anonymous 

I think I was able to get something close to what you are lookin for.  The  categorization of users and user count by categorization will both follow the slicer selections.

Categorization = 
VAR _Y = 
    CALCULATE(
        SUMX(
            DISTINCT('Table'[user id]),1)
            ,'Table'[includes attachment?] = "Y"
    )
VAR _N = 
    CALCULATE(
        SUMX(
            DISTINCT('Table'[user id]),1)
            ,'Table'[includes attachment?] = "N"
    )
RETURN
SWITCH(
    TRUE(),
    _Y = 1 && _N = 0, "Always",
    _Y = 1 && _N = 1, "Sometimes",
    _Y = 0 && _N = 1, "Never"
)

User Count = 
CALCULATE(
    DISTINCTCOUNT('Table'[user id]),
    FILTER(
        VALUES('Table'[user id]),
        COUNTROWS(
            FILTER(
                Categories, [Attach Yes] = Categories[Y] && [Attach No] = Categories[N])
        ) > 0
    )
)

I did add a categories table for use when grouping the user counts.

I have attached my sample file for you to look at.