try this catalog

some other errors:
 1) <variableAgg name="dsp_band_1" /> names the field you want to aggregate
 2) coordValue="1999-01-01T08:24:17Z", not coord="1999-01-01T08:24:17Z"

<catalog xmlns="http://www.unidata.ucar.edu/namespaces/thredds/InvCatalog/v1.0";
         xmlns:xlink="http://www.w3.org/1999/xlink";
         name="URI-THREDDS OPeNDAP Data Server" version="1.0.1">

 <service name="opendap" serviceType="OPENDAP" base="/thredds/dodsC/" />

 <dataset name="Pathfinder AVHRR Northwest Atlantic Aggregation (6km Declouded)" 
urlPath="AVHRRAggregation.nc" ID="AVHRRAggregation">
    <serviceName>opendap</serviceName>

    <netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2";>

       <dimension name="time" length="0"/>
       <variable name="time" type="String" shape="time" >
          <attribute name="_CoordinateAxisType" value="Time" />
       </variable>

       <aggregation dimName="time" type="joinNew">
          <variableAgg name="dsp_band_1" />
          <netcdf 
location="http://dods.gso.uri.edu/dods-3.4/nph-dods/Pathfinder/Northwest_Atlantic/6km/declouded/1999/1/f99001082417.hdf";
 coordValue="1999-01-01T08:24:17Z" />
          <netcdf 
location="http://dods.gso.uri.edu/dods-3.4/nph-dods/Pathfinder/Northwest_Atlantic/6km/declouded/1999/1/f99001181038.hdf";
 coordValue="1999-01-01T18:10:38Z" />
          <netcdf 
location="http://dods.gso.uri.edu/dods-3.4/nph-dods/Pathfinder/Northwest_Atlantic/6km/declouded/1999/1/f99001195049.hdf";
 coordValue="1999-01-01T19:50:49Z" />
          <netcdf 
location="http://dods.gso.uri.edu/dods-3.4/nph-dods/Pathfinder/Northwest_Atlantic/6km/declouded/1999/1/f99002081244.hdf";
 coordValue="1999-01-02T08:12:44Z" />
       </aggregation>
    </netcdf>
 </dataset>
</catalog>