Call Agent switching Teams

You can create a measure to use as a filter on the slicer visible. 

The basic pattern is something like

Agent is visible =
VAR MinDate =
    MIN ( 'Date'[Date] )
VAR MaxDate =
    MAX ( 'Date'[Date] )
VAR AgentStartDate =
    SELECTEDVALUE ( 'Agent'[Start date] )
VAR AgentEndDate =
    SELECTEDVALUE ( 'Agent'[End Date] )
VAR Result =
    IF (
        AgentStartDate <= MinDate
            && (
                AgentEndDate >= MaxDate
                    || ISBLANK ( AgentEndDate )
            ),
        1,
        0
    )
RETURN
    Result

You may want to tweak the logic a bit to handle cases where someone leaves part way through the year. As it is they would still be included, but you might want to exclude them.

Add the measure to the slicer as a filter, set to only show when the value is 1.

You could create another calculation item like

Agent is visible calculation item =
VAR MinDate =
    MIN ( 'Date'[Date] )
VAR MaxDate =
    MAX ( 'Date'[Date] )
VAR AgentStartDate =
    SELECTEDVALUE ( 'Agent'[Start date] )
VAR AgentEndDate =
    SELECTEDVALUE ( 'Agent'[End Date] )
VAR Result =
    IF (
        AgentStartDate <= MinDate
            && (
                AgentEndDate >= MaxDate
                    || ISBLANK ( AgentEndDate )
            ),
        SELECTEDMEASURE ()
    )
RETURN
    Result

and use that as a page level filter