If your group skill is not maxed(or close to), donation
Yes, group skill apply to all ninjas, and if you break down the cost per stat point, group skill tend to be cheaper (remember, 3000 coins per contribution, for a 40 contribution skill, for example, it's 30k coin investment on EACH of your ninja(120k total) for the stat bonus on EACH of your ninja, how many upgrade does it take for you to get the same stat on one ninja via equipment upgrade?
You should throttle near completion because you do earn contribution through convoy etc, which can be used to max it out lower but if you are waiting for your group to upgrade anyway....
The wheel is a poor use of contribution until your group is at least lv 7, at which point the group growth is too slow for skills cost to even be relevant(also the cd per skill is so long so you can't spend much there anyway)