When you first start your co-op you can have five members. To find members you can look for players in the rankings list (icon with three people at the bottom of the screen). You can also find members by looking through the thread Cooperative Search on the Big Farm forum. There are two ways to add members. You can send an invitation to a player or you can accept their application when they apply to your co-op. When you have all member spots filled you can do the research Recruitments. You will need to get your other members to particpate in research with farm dollars or researchers can be bought with gold. Once research is complete you will be able to add up to two more members to your group.