Thrive will add the organization that you will work under. Once the organization is created, groups can be added within that organization to keep users together.
Organizations help admins collaborate together on the same content while groups allow admins to segment users. Users can belong to more than one group as well. Use groups however you would like.
Some admins prefer to keep it simple and only have one group that just reflects the organization’s name. Others prefer to segment users in many ways which requires very specific and deliberate naming and creation, respectively. Since users can belong to more than one group, this can help make scheduling an easier process.