Hosted.IM domains can be configured to authenticate against a custom backend. If you are able to build a basic HTTP/HTTPS API to your user base, you will be able to use hosted.IM without the need to replicate your userbase.
The http request needed is very simple and requires only a GET with 4 parameters: Username, Password, Domain and Secret. Secret is provided by hosted.IM backend when you enable the feature.
Inside a given plan, users are counted as number of different username used over a calendar month.
See the blog post presenting the feature and details about how to implement it: User authentication against your company database.