When I do a group ride on Zwift can I use erg mode to stay at the same power or no?
not through Zwift, but you can control your trainer using a head unit or other app potentially
Not in Zwift but control your trainer via a Garmin Edge.
Some ride leaders do that when their legs are tired. 
One even admitted to this here.
Be aware that your pace will be somewhat like a robopacer so other people in the ride might not be pleased.
You can’t have the Garmin produce power it can only control the resistance, so the ride leader still needs to do the work.
I didn’t say the Garmin produced power, just that it sets the power.
If I remember right it was you who mentioned it at first.