EDIT: Thanks for informing me that the discrepancy was because lower mining skills mean longer mining, rather than because anything has actually changed. Regardless, I still think it would be better if it were to work this way, so consider this a suggestion. :)
It used to be that when I mined the same rock as a bunch of other people, I would get a bonus solely proportional to how many others were on the same rock, whether I was first or last to get there. Now, it only gives me bonuses for anyone who joins in after I do. From what I've heard, a lot of the elitism in mining centers around who gets to have first shot at the rock. If this change were to be reverted, the focus would be removed from "who gets there first" and put back on "how many"; i.e., it would encourage the whole community thing, which is what you want. As it is now, people may be discouraged from mining in groups because being the last one to a rock only means they'll get fewer chunks, rather than getting more because there are more people there (which is probably why a lot of people started mining in groups in the first place).
Please consider changing the balance of bonuses back to the way it was. I think it will be good for both the individuals participating in group mining and the community attitude towards doing so.