Please note that the information regarding the Harvesting Classes is not accurate. However, until we can gather more information and exactly how they've changed, we're opting to keep what's here, here for now.
You can advance to max level alone if you so desire.
The interesting logic behind these Harvesting Classes is that they complement each other very well, which will encourage people to group while harvesting. Why would you want to group to go harvest resources? The designers have given this a lot of thought, and made it so that IF you are grouped with complementing harvesting classes, you will receive a bonus to your harvesting. This includes the possibility of increasing your chances to both harvest more efficiently as well as harvest rarer resources.
Keep in mind that grouping is more than just getting a bonus to harvesting. Harvesting and Crafting Classes have roles, just like Adventuring Classes. Some activities will require you to group with other harvesting classes, just like certain monsters require more than one person to kill. There will be interdependence, but it will not be required to advance in game.