The way i upgrade is: group level > group skills > group wheel > group activities. It's up to you if you upgrade skills or wheel first, skills give a small power boost while wheel will result in few coupons more on average. I'm leaving group activities for last upgrade, because the only thing that changes here is a small boost in gold- not much in comparison to power or getting cp. Level those things up in a priority you choose as soon as you can, no point in stacking up group contribution until you can level all things up at once. In order to upgrade group level, you need to upgrade all others as well to the same lvl, so it's only a matter of order.