Calendaring tool for distributed team and subcontractors

Hi NNCG braintrust,

I do part of my consulting for Ekstrom Alley Clontz, a virtual firm with one full-time managing partner and the rest of us as on-call project managers and do-ers. People use a mix of Google Apps, Microsoft, and other tools to manage their personal and work calendars (not everyone in the same email domain). 

Are any of you using a tool that without too much complexity:  a) keeps a live look at others’ calendars without forcing them to create a new email/calendar account, and b) maybe contains a central meeting/admin calendar for everyone to see? Ideally without having to buy into a full project management system. My guess is there are such tools for business but also for things like community sports teams, but I’m having trouble identifying them. 

Thanks for any advice – I owe you drinks at least virtually