Hi there,
As a sound engineer, I always use the AUX sends for the monitors, as opposed to the groups, which I normally use for grouping channels sent to the main mix. The monitor output on your desk (presumably the USB model) are for studio monitors, as opposed to stage monitors.
Having a quick look at a pic of the desk on the net, I'd say your best bet is to use the Aux 1 output for monitors. All you then have to do is send whichever channels you want reverb on to the DFX channel, and turn the Aux1 knob on the DFX channel up as desired. this way the effected (wet) signal should appear in your monitors.
Try that and see how you get on!
Lorry