Reddit reviews New TOEIC Official Test Exam: Toikku tesuto shin koshiki mondaishu (2CD) [Japanese Books]

My initial advice is to do everything in your power to avoid this. Unless your current instructor is a serious hotshot. Push hard to outsource the teaching/curriculum to a well known vendor. They simply have access to a helluva lot more resources (e-learning, correspondence courses, etc.) and know-how than you'll ever know. It won't come cheap.

Now that that's out of the way, let's discuss reality.

First off is the admin side. You'll need to confirm just how often employees will be able to attend lessons, how often you'll be able to hold lessons, and are there any rules/regulations regarding attendance, achievement, etc. You'll also need to make sure to have meeting space booked and ready to go beforehand, but you're probably already doing this with your regular teacher. You'll also want make it known to management what kind of results they should expect after 1yr, 2yrs, 3yrs because it's bound to be unrealistic.

As for curriculum itself, I would aim for a combination of workplace english, and TOEIC prep (vocab, grammar, and test taking strategies). How you weave these together into a cogent program is the tricky part.

My coworkers and I disagree on this, but I would recommend avoiding TOEIC prep with the lower level groups (under ~400) at first. I prefer them to focus on one thing at a time, get comfortable with an all-English environment, and interacting with each other as much as possible. Throw TOEIC at them too early, and they're bound to be overwhelmed and discouraged. Popular materials here are Business Venture, Passport to Work. Four Corners, Interchange, and Smart Choice are nice for general English as well.

Once you get into the higher levels, you can start mixing and matching. For that, I would want some kind of test prep book with Japanese support:

https://www.amazon.com/New-TOEIC-Official-Test-Exam/dp/4906033342

Obviously, you can use this for mock tests and whatnot. But you can also use the listening and reading tasks as a springboard to other communication tasks.

It's also nice to have a grammar book geared for TOEIC, and possibly a vocab book. There are countless materials here. You'll want to put the guys on a self-study schedule for these, and spend a bit of time on it either each lesson or every other lesson. A possible flow for one of these lessons might be 20-30min grammar task review and practice (hopefully interesting). ~60 min of communication practice with/without TOEIC materials. 5-10min smoke break somewhere in there. Then use the remaining time for whatever (vocab review, relaxing discussion, work-related help).

Hope this helps, but the first advice still stands. Try to get out of it.