We already have post regarding regular votes (Vote Early, Vote Often (Seriously)), but I want to cast a special light on close votes. Private beta is about making the scope of the site more clear. Thus, trying to close questions you think are not a good fit is essential.
Don't hesitate to vote to close questions you think are a bad fit. People will either agree or disagree, by joining your close vote, abstaining or challenging you on meta. Either way every discussion about what's on scope and what's not on scope is a good discussion. It sharpens the scope and sets precedents we can work with.
Remember you can access the review queues earlier and have the privilege to close vote as long as we are in private beta even without those 2k reputation. It's a basic privilege in private beta. Also remember if you bring something to meta and it's not well received, you do not lose reputation on meta for down voting or being down voted. So vote on meta on those on-topic questions.