Organized GNW pack distribution sure helps, you can easily come up with a point value system between donation, GNW participation and potentially even other participation matters(granted, some things are kinda impossible to track, I would really like, for example, know who is doing ranked battle and who isn't, because we are having difficulty getting the 180 ranked per weak activity done(except last week due to the monthly event) and therefore doing ranked when it is "useless"(most player are pretty stuck) IS contributing to the group.)
But sadly, that can't be tracked. I would really like a chart of who is doing what in the activities page...
On the other hand, I see very little point in forcing player to donate when they have already maxed their group skill. Quite frankly, if a player have maxed his group skill, chances are he is NOT the reason the group skill(and other things) isn't higher (and thus requiring more donation), it is far more likely that it is the fault of other players, unless of course the group have very low amount of active players, in which case the per player contribution required to level up the group is significantly higher.
Personally, i care very little about donation itself, more for contribution. If a player makes an effort to get the contribution for free via convoy/plunder, GNW and group trial, it grows the group just the same. Just like ranked, not everyone bother with group trial... While I understand some people can't make GNW/convoy hours and this would be unfair as donation would be their only option compared to others, but those are far and few in between if they actually picked the server of the right time zone. And if they didn't, that's really their fault.