There's actually a very simple way of doing this, though it's using an advanced entity.
The entity is question is 'trigger_add_or_remove_tf_player_condition', i'm unsure if this is supported in the default fgd, but basically you'll want to change the attribute to '??? damage vs players', the duration to '-1', and the value to 0. This will make it so players will deal no damage to other players while inside the trigger volume.
And here's the view with smartedit off, in the event it's not in the fgd.
If players are still dealing damage while in the trigger volume, try changing the 'value' to 100 and see if that fixes it.