How do I allow site users to add other site users in a B2B site?

To allow a site user to add other users on the site, you need to make sure that the user is a member of a site group that has permission to create users. The permission that controls this ability is called "Can create a new user."   You also need to make sure you have a link to "Profile" in the navigation of your site.  The profile is found in the Standard Items.  Once the user clicks on the profile link, they will see a tab called "Users." There is an "Add User" button on that page.