The default Microsoft SAPI speech recognition engine is intended for a specific user, not to recognize speech from multiple users.
If you want to try regardless, you'd need to use audio routing software, E.G. "VoiceMeeter" (despite the name, this software is not related to VoiceAttack, nor is this intended as an endorsement) to route the output from your Discord application to a virtual recording device together with the output of your microphone, which the speech recognition system would then need to configured to listen to (the latter can be done using the "Windows Speech Recording Device" option on the "Recognition" tab of the VoiceAttack options window).