Are you planning on targetting a specific group of people - such as site owners of the topic your site covers? or a particular age/interest group?
And would this be a few hand-written (so to speak) invites, or mass-mailing?
The content could be a simple introduction of yourself, tell people about your site and what makes it interesting and unique, maybe even offer something to pique their interest (a free "subscriber" account for 3 months or some other similar "extras" for signing-up?).
What's your site URL - maybe a few of us here could help you draft something