Members who are added by a manager and members who join the group are considered the same for your total membership number - both show up in your database, and both are considered 'Members' of your group.
The difference lies in the fact that people who are added to the database without being required to fill in your membership form (if you have created one), will not be able to use private pages of your group, post to the forum or log in to change their membership details without a direct link in an e-mail from the manager. All these things require a 'GroupSpaces Account'.
A GroupSpaces Account also allows the user to use the same login details to join and any number of groups, and have them all accessible from their 'My Account' page.
Members wanting to manually join the group are directed to the membership form and have to fill in any required fields before they are added to the group. This will add them to your group database, add data to any custom fields and also create a 'GroupSpaces Account' for that e-mail address.
Members can also be 'Mailing List only' if they have signed up via an embeddable widget from the 'Website Integration' page. This is exactly the same as if a manager added them to the database.