For buffs on groups: only buffs which immediately affect users stats get applied to group stats. For example, Constitution buff does not immediately affect your stats, it's calculated in combat. Only the group leader get the combat buff calculations. If you have a 5 person group and all group members have Constitution running, only the leader's Constitution is calculated. If we're talking about a buff like Enchant Weapon, that immediately affects a user's stats, so it would be calculated for every member of your group.
In your NMV example, same applies as Constitution. It will only calculate NMV on your stats then add them to the group stats. NMV will NOT be calculated on your group members.