Best Way to Mass Import Users into WordPress

I've created a web app, sorta setup with only the admin dashboard of WordPress and without too much programming skills. Now, it's going to be used in a much larger environment than I have ever dealt with before. Users will not register by themselves on my site and I now have the need to add between 900 - 1,000 users into WordPress (containing 2 different user roles) pretty soon.

Since I don't really have much knowledge in this field, I will need assistance from a WPMU Dev staff member with this. Also, I have no need for using email addresses with these users. My current plan is to just put random email addresses at my domain (because no email addresses are actually at that domain) — but if someone has a better suggestion, I'd be happy to hear about it. I have the names and usernames of the users that I would like to add into WordPress.

Can someone provide me a suggestion with walk-through of a fast, efficient (and safe) way of adding this huge amount of users into WordPress Please respond ASAP. Thanks in advance!