Theoretically this could be achieved by creating two custom actions in SharePoint. The first custom action would hide the existing Manage Users menu item from the Site Actions menu. The second would could add an Invite User menu item to the Site Actions menu item.
(SharePoint Custom Actions
http://software.sharepointsolutions.com/Community/p/164/398.aspx#398
(Hide Custom Action Element)
http://msdn.microsoft.com/en-us/library/ms414790.aspx
Here is the definition for the custom action you would want to hide...
<CustomAction
Id="SPSolutions.SharePoint.Delegation.DelegationSiteActionsMenuCustomizer"
Location="Microsoft.SharePoint.StandardMenu"
GroupId="SiteActions"
On final thought, you will probably want to limit the Invite User menu item to Site Collection Administrators, Site Owners and possibly Delegated Administrators. To do this will require a bit of coding. To do this you will have to write a .NET Web Control and use the ControlAssembly and ControlClass attributes of the CustomAction.