You want to use logical
ands and
ors.
So you must change a couple things
(Fields!VTL_EMPTimeStamp.Value)>=(Parameters!StartDate.Value)
is to be replaced with
(Fields!VTL_EMPTimeStamp.Value >= Parameters!StartDate.Value)
or globally,
iif((Fields!VTL_Emp.Value="TRUE") AND
(Fields!VTL_EMPTimeStamp.Value>=Parameters!StartDate.Value) AND
(Fields!VTL_EMPTimeStamp.Value<parameters!enddate.value),1,0)>
or
iif((Fields!VTL_Spouse.Value="TRUE")AND
(Fields!VTL_SpouseTimeStamp.Value>=Parameters!StartDate.Value)AND
(Fields!VTL_SpouseTimeStamp.Value<parameters!enddate.value),1,0)>
or
iiF((Fields!ADD_Emp.Value="TRUE")AND
(Fields!ADD_EMPTimeStamp.Value>=Parameters!StartDate.Value)AND
(Fields!ADD_EMPTimeStamp.Value<parameters!enddate.value),1,0)>
or
iiF((Fields!ADD_Family.Value="TRUE")AND
(Fields!ADD_FamilyTimeStamp.Value>=Parameters!StartDate.Value)AND
(Fields!ADD_FamilyTimeStamp.Value<parameters!enddate.value),1,0)>
Lines are here to make the expression clearer.
More simplifications can be done.