If your Google Apps running on your domain, you can add Wave by going to your admin control panel, clicking ‘Add more services’, finding Wave then clicking ‘Add it now’.
What’s more you can give your Apps users a nice friendly URL for Wave, for example ‘wave.mydomain.com’ in the same way as the other Google Apps. Simply create a CNAME for ‘wave’ in your domain DNS records, then from Admin console, go to the Google Wave Settings page and click the ‘Change URL’ link. Enter in your details and you’re away. (More detailed instructions are available from Google!)
Now if only anyone else I knew used it!