Members are people you have actively invited to be a part of your organization on the ‘Members’ section of Organization Settings. They can create conversations (rooms) attached to your organization, and are able to add and remove members from organization rooms, among other things. You are billed for each member of your organization.
Guests are people who have only been invited into rooms attached to your organization. They have limited functionality available to them – for example they cannot add or remove users, move or quote messages, and they cannot create rooms attached to your organization. You are not billed for guest users.