Password for first-time visitors?

I am setting up a blog and want to permit access only to my company’s clients. Is it possible to give them a one-time password or log-in that they can use the first time they visit the blog? I don’t mind if people who are not my clients access the site as well (if they have been told about it by friends or colleagues), but I don’t want it to be available to just any random person.

I know I can ask people to register when they visit the site, but I might not get around to approving new users very often. Is there a way I can accomplish this by giving people a code or password they can use just for the first time they go to the site?