I think a simple good way to encourage people to gather/craft in groups is either allow for more items to be crated/gained from grouping or have specific recipes and materials that only groups can gather.

Of course then you have to take the market into account and people would be upset if they couldn't make a Pwnzage-hatchet axe with just their Blacksmith or Carpenter.

I think group-crafts/gathering could encourage people to at least participate it doesn't need to boost exp or anything.