For now, you could try sharing each person's calendar.
You might want to consider voting for
Bug 36322 - ics freebusy feed and/or
Bug 37836 - RFE: external web access to multiple users' free/busy times, viewable on a single page
The first would allow users to access free/busy info
(only) in .ics form. You could then aggregate free/busy by subscribing to multiple "WebCal" feeds e.g. using iCal. It could probably also work with Lightning, Outlook, or certain web-based services.
The second would directly create an aggregated free/busy view for multiple users, in a web page.