Hi Mike,

Is there no Invite class in XWiki Workspaces?

How do admins in a Workspace add users just to that workspace?

Assuming you are an admin in that space : click on the "Administration" link in the panel on the left, then click on "Space members" in the panel.

Now you can invite existing users to be admins, authors & readers in that given space.

Check out : http://workspaces.xwiki.org/xwiki/bin/view/Features/StandardWorkspace & http://workspaces.xwiki.org/xwiki/bin/view/Features/SpaceMembers 

Guillaume