First off, how have you gotten these email addresses to send to? Are they already customers? Just be aware of the various spam laws in place and how to avoid them.
The best email has value. What is the problem that your beta is trying to solve? For example, say your beta is an online service to maximize what fishing lures to use for your next fishing expedition (way far out man!)
Then your email should be phrased to show the problem and potential value of a solution. So your subject would be something like, "Why choosing your fishing lures matter". A subject that immediately shows there's value in your email.
In your email body, you want to describe the problem and why it matters. Then you need to show the value of the solution. Then propose your beta, or link to another landing page which has more detailed information and a call to action.
"Choosing the wrong fishing lures for where and what you are going to fish can ruin your next fishing vacation."
"Effective choices of fishing lures based on location and season can maximize your catch and bring excitement each time you head out with your fishing pole."
"LuresRUs is creating the next innovative way to provide the best fishing lures for your next trip. We crowd source the knowledge of fishing experts around the world to suggest the best lures and baiting solutions. And we're giving you the sneak peek."
Call To Action:
"Get Your Private Beta Invite Now"
By the way, I have no idea how to fish. I just wanted to choose a scenario that is completely different from my own startup to provide a non-biased suggestion. My apologies to any avid fishing experts if I used the wrong terminology.