How about using the very same invitation email? Let me write down my idea - please feel free to comment:
We are a group of users who are starting a higher-level (graduate-level and above) physics site, called Physics Overflow, outside the Stack Exchange network. In addition to a Q&A section, we will also be releasing a "Reviews" section, in which all papers from ArXiV will be imported, for our users to review. Eventually, we will extend this to journal databases, conference proceedings, important conferences and seminars, and so on.
Physics Overflow has graduated from its private beta phase, and is now a full grown website. Hence, we wish to invite all those interested in posting graduate-level questions, giving answers to such questions, etc. to join this website.
If you are interested in participating on Physics Overflow, you may find us here: http://physicsoverflow.org/.
Physics Overflow (Company)