Mute groups are not created by the controller you use. The machine that the midi data is being sent to needs to support mute groups. The only way u can "fake" mute groups is by leaving your pads set to gate mode and then using a daw/sequencer that you can set the value for the gate length.

+1

it would be entirely achievable, but none of them do because in most cases the sampler provides the functionality

In theory, it is possible. But you'd probably need to write a patch in something, say in Max for Live or something. The controller won't do it itself. It's a good idea though