I have quite a few gripes about Farmville Co-op jobs, but they are the best way for people with small farms to get ahead quickly and get bonus points. How else can someone with a hundred or 200 plots regularly get 1000 XP in a day or so without a lot of planting and harvesting?
The biggest problem initially was that many neighbors and friends could not join jobs of other neighbors and friends. This seems to have been improved.
1- All the jobs use crops that do not have a high point value. If you plant your whole field with grapes (2 XP per day) to do a Co-Op job, you can lose, say, 400 XP that you would have gotten had you planted peas (3 XP). But if you have enough partners, you don't need to plant a lot of grapes or anything else. Everyone gets the same XP bonus for a job, no matter what their contribution was, as long as they harvest before the job is done. The best sort of job management allows as many people as possible to join. If you are leading a long job (4 days 21 hrs) and have enough neighbors, you can afford to plant only part of your field for the job, and then wait a few hours to allow others to join. Meanwhile, plant a fast crop like blackberries on the rest of the fields. If there are not enough people joining, you can always plant up the rest of your fields for the job, allowing enough time to get the gold medal.
2- Don't start a job if you don't have a lot of Facebook friends who play Farmville. Only friends of the group leader can join your job.
3- Don't join a job that is very late unless you can add enough to finish it. Chances are, if nobody joined after a day, nobody is going to join - the leader hasn't got enough neighbors and friends who play farmville.
4- There is always the possibility that someone will not harvest in time, which can sabotage the job. To get insurance against this, each person can plant more than is required, even if it doesn't register as seeded. If your job requires grapes, all the grapes harvested while you are in the job will be counted, regardless of whether or not they were registered as seeded, up until the maximum. Of course, this means that those who do not harvest in time may be shut out. And you can plant, for example, grapes, and then join a job afterwards. Seed more grapes, and then you can harvest the ones you planted and the harvested grapes count for the job. The seeded ones will not be counted when they are harvested, but still, it means you can actually (in theory) finish a job in an hour, if everyone joins the job after their grape (or tulip or whatever) crop is ready.
5- If you can coordinate a team of people who do jobs together, you will all start together and finish quickly, minimizing the wait between jobs.
6- Not everyone in the job is necessarily listed on the main job page display, because the space is limited. Click the "Friends" tab to see all the people in the job.
And remember - it really is only a game. :-)