This may sound a little convoluted at first, but here's how I would set it up:
I would set up the usability lab as an equipment resource. (If you have multiple labs that can be booked separately, then set each one up as a separate resource. Create a task group for each company. (Each task would be a testing session.) In the task view, you can see all the companies' testing schedules at once (with a bit of scrolling, perhaps).
In the resource view, you can display the resource load graph. This will show green when the lab is in use and pink where the lab is overbooked.
In the calendar view, you can customize the work schedule for the lab. This will allow you to indicate when the lab is closed for maintenance or internal use or something like that.
Hope that's enough to get you started. You can download OmniPlan from our website and try it in demo mode for up to 20 tasks, or request a free 1 day full license from our online store.
Feel free to post more questions here, or send email straight to our support ninjas at
omniplan@omnigroup.com.