No. If they don’t have a usrname account at WordPress.com they cannot use the “;ike” button. the same is true of Facebook and everywhere else I’m aware of ie. you need to be a member to use internal functions.
You’re welcome. It’s easy to get a username account only. No one has to register a blog. There is a “username only option” and al that’s required is an email address (I recommend a throwaway one that’s not associated with a gavatar account or another blog) see here > http://en.support.wordpress.com/register-a-blog/