You can invite anyone to Google Wave. Google Wave is now open for all. Anyone can signup to Google Wave. Google opened Wave as part 2of Google Labs on 18 May, 2010. Now you can add anyone to new or existing wave even if he is not in your contacts. An invitation will be sent to him to join Google Wave with the description of wave. Just follow these steps.
1. Create a blank wave or use an existing wave
2. Click on ‘+’ sign to add any email address as a participant to wave
3. Enter the email address and click on Invite via Email button