"What do you need?" is certainly not enough and will lead to you receiving a lot of generic suggestions as your research group attempt to second-guess you and give you the answers they think you're looking for.
Instead, focus them on common tasks - searching for a colleague's contact details, administering their employment - and look for ways to improve the experience by adding missing information and shortening search and navigation times. Ask them to prepare a list of problems to bring along to the meeting so you can discuss and prioritise the most important problems.
To find out what's missing, ask them which tasks in their work take longest to complete. Aggregating these will give you an indication of where to focus your initial efforts to help the broadest range of people.
Designing useful homepages is not easy given the need to satisfy your stakeholders, particularly when they don't form the bulk of your users. This article on designing useful intranet homepages explains some of the pitfalls.
I'm not sure I'd introduce design probes until a later stage, once I have identified some problems to research.