Create calculated table from two tables

EnderWiggin Try below. PBIX is attached below signature.

Username Sum Quantity Top 3 = 
    VAR __BaseTable = 
        SUMMARIZE(
            'User Activity Table',
            [UserId],
            "Sum Quantity", SUM('User Activity Table'[Quantity])
        )
    VAR __Table = 
        ADDCOLUMNS(
            __BaseTable,
            "Username", MAXX(FILTER('User Table', [Id] = [UserId]),[Name]),
            "Rank", RANKX(__BaseTable, [Sum Quantity],,DESC)
        )
    VAR __Result =
        SELECTCOLUMNS(
            FILTER(__Table, [Rank] <= 3),
            "Username", [Username],
            "Sum Quantity", [Sum Quantity]
        )
RETURN
    __Result