Speedy Tertle has an English speaking guide. I dove with Eric there a few years ago. He's still there, I hear, and a pretty cool guy (maybe even 5513 poster above!). He set up some nice dives for me.
Your choices on Saipan are limited unless you speak Japanese, Korean, or maybe Chinese. Apparently All American can help you out as well. There are over 40 shops on the island (which takes about 25minutes to drive across on the roads), but as far as I know for English-speaking shops those are your two choices. You could try Seashore Diving Office as well. It's a Japanese shop but a couple of the guides can speak pretty good English. Their service is impeccable and prices reasonable. You'd do well to contact whoever you want to work with in advance. When I dove with Speedy, they had only three guides working so if you are talking about a slew of divers, you might run into trouble accomdating them all. If you decide to contact Seashore, keep your English simple and use the phone.
サイパンのダイビングショップ「シーショア」のＷＥＢサイトへ ようこそ！ １-６７０－２３４－５５４９ １－６７０－２３３－５９０２
Speedytertle Dive Shop: Saipan's Premiere Dive Shop
I hope you can get a chance to dive. Saipan is really a wonderful place for it.