Gee is, as usual, quite correct.

I would also add that if it is not always possible for people to use Outlook, then resort to something a little more old-fashioned but still workable.  Ten people is a lot of people and everyone's diaries are quickly filled so more than one date is useful:

Start with most important person first (i.e. your boss)
Consult boss's diary and pick a few dates and times
Email everyone who should attend meeting with selection of dates and times and give deadline for reply
Follow up any stragglers at the deadline
Collate and choose final date and time
Circulate final date and time to all concerned
Make sure this is in your diary and your boss's diary
And somewhere along the way, don't forget to reserve the meeting room and refreshments.