List users by last name, paginate by the first letter in the last name, A-C, D-F, G-I etc?

Does any of you know how to produce a list of users, sorted by last name, and paginated based on the first letter of the last name?

I want to produce pages with users who’s last name starts with A-C (page one), D-E (page two) and so on. I have over 2000 users, so to show all on one page won’t be ok, and I don’t want to have numbered pagination as that makes it tedious to browse.

I’ll be serving it out as JSON, to a mobile web app, using the “JSON API” plugin, and I’m also using the plugin “S2Member”.