The goal system for goals in which you can have others join to help you is currently, in my opinion, slightly flawed. Presently, you get coin payouts based on how many songs you played towards the goal. This makes sense in principle - the people who put the most in should get the most out, right?
Here's the problem - I was at work today, and got invited to join a friend's Dave Grohl goal - I join, and see a bunch of other friends in the group. We start talking about who has what DLC, and I'm excited to get home and start cracking on this goal (which has a nice coin payout to boot).
I get home, boot up, hop on Facebook to see what needs to be done...
And see the goal is 2 songs from completion. By the time I get into the game, the goal is complete. I have been awarded 0 coins for the goal, and it is no longer available for me to select again, since it's considered complete.
The worst part is, I could have completed this goal by myself; I had enough DLC to do so. So I start looking at all the other goals I had joined that day, and sure enough, most were about to be completed. I was able to salvage a few coins from some of the others, but it made me wonder how many people were going to join goals in the future once they realize that if they don't play in them fast enough, they will forever lose the coin bonus associated with it.
What's worse - people can join your goals, and complete them, without you ever having an opportunity to stop them. If I'm at work, and am going through the goals, and start a few so that when I get home I can get cracking, other people can join that goal and complete it without my consent, and I get 0 coins and no chance to replay the goal. That's an issue.
Now, I get why it's like this: it prevents people who had no songs for the goal from free-loading and getting coins for doing nothing. But there should be some minimum payout for anyone who joins the goal - say 10% of the total. At the least, someone who contributes nothing should be able to do the goal again (it won't mark as complete).
Bottom line is that I may be less inclined to join group goals for fear that I won't get a full payout, or that it will be completed before I get a chance to play it. I certainly won't bother responding to requests at work (which means I might miss out on groups that fill up quick).
Note: This doesn't apply to "community goals" - I know that that is a base payout for everyone, regardless of participation level.