Repartition by max activity

This works if you use columns from the dimension table rather than the activity table

Count members with main activity type = 
VAR CurrentType =
    SELECTEDVALUE ( Activities[type] )
VAR BaseTable =
    ADDCOLUMNS (
        CALCULATETABLE (
            SUMMARIZE ( activity, Users[account], Activities[type] ),
            REMOVEFILTERS ( Activities[type] )
        ),
        "@sum", CALCULATE (COUNTROWS( activity ) )
    )
VAR PartitionedTable =
    INDEX (
        1,
        BaseTable,
        ORDERBY ( [@sum], DESC ),
        PARTITIONBY ( Users[account] )
    )
VAR Types =
    GROUPBY (
        PartitionedTable,
        Activities[type],
        "@users", SUMX ( CURRENTGROUP (), 1 )
    )
VAR Result =
    SUMX ( FILTER ( Types, Activities[type] = CurrentType ), [@users] )
RETURN
    Result