Which part of the user properties window allows for adding or removing users from security groups?

The Member Of tab is specifically designed to manage the group memberships of a user within the user properties window. It displays a list of all the groups the user is currently a member of, and it provides functionality to add or remove the user from various security groups. This tab is essential for managing user permissions and access controls, as it directly impacts the user's rights and access to resources based on group affiliations.

In contrast, the General tab typically contains basic information about the user such as name, contact details, and account status, but does not handle group memberships. The Security tab deals with permissions assigned to specific files or folders rather than managing user group memberships. Lastly, the Permissions tab, when applicable, relates to granting or denying access control settings for a given object but does not pertain to user group memberships. Thus, the Member Of tab is the correct choice for this task.