Sometimes I need to send emails to just some of the users on my mailing list - is this possible? I'm using the Starter version.

We don't have easy interface to do that but:

You can create a new list and subscribe these users to a list and then send your Newsletter to this temporary list... that's not an easy solution if it's just for few users.

Or you can simply insert the e-mail address of the user in the preview screen and send him a test. Even if it's a simple test, the receiver will receive the normal Newsletter.