If you all have Gmail & you are happy with that use it.
Some things to bear in mind…
There are a several ways to share calendars, people can either subscribe to a public calendar & see events that they cannot edit like this one…
https://www.apple.com/downloads/macosx/calendars/usholidaycalendar.html (click download to open in your Calendars app).
Or users can publish to others privately & allow editing of the events with some authentication controls.
Google, iCloud, Fruux and many other web services support both options, however the second type usually has some limitations.
Here is iClouds limitations (100 users per calendar)…
http://support.apple.com/kb/HT4489
Google will have similar limits on 'personal' gmail accounts.
If this is a 'company calendar' do you need to have full control over it? e.g. Do you need to be able to close accounts or fix settings for users etc.
Google have the 'Apps for business' specifically to give you control.
http://www.google.com/enterprise/apps/business/
That can allow you to also share other things like contacts or Google docs etc. You can also use your own domain name with apps for business so that things can be configured with "mydomain.com" when you setup the devices. You can also have email@mydomain.com etc, which looks better than some-randomstuff@gmail.com.
Whatever you choose just bear in mind that any free service may change with little warning - even when you pay Apple it doesn't guarantee it will always work. They have gone through many tweaks with iTools, .Mac, MobileMe, iCloud - Apple often abandon many users on older OS's, so I cannot recommend them for anyone who wants longevity or business use.