This is something I have been looking to find for some time.
with no luck I am afraid. I am currently using an adding we have had for years called Workgroupshare that does interact with Outlook but does not play well with other OS such as Ipad/Iphone/Blackberry. The resulting mix of Sync's from each is creating a ton of duplicate and "old-removed" entries that suddenly reappear with no way to figure out whose laptop or phone did the trick.
The ideal solution would work as both an Outlook add-in plus being able to run on Apple IOS/Android/Blackberry etc. A multi user appointment scheduler with the preferred format the same as an Outlook calendar. Updates (synchronizations) should be automatic at timed intervals and the ability to display each user in a different color (up to maybe 10 or so) would be a major plus.
Importantly, it needs to be able to track which user made what changes and when so that erroneous changes or entries could be traced back to the source. Historical data for at least 2 months in the past as well as 2 months or more in the future would need to be included in each sync. The items stored would also need to be able to link or store internally various small files like itineraries, reservation receipts, etc.
Outlook 2013's shared calendar was supposed to do all this .. and does ... as long as every device is using Outlook 2013. Not so good on a smartphone.
If the only option is a true webhosted Calendaring service that might work even if it does not link with Outlook's calendars, as long as it has keeps a local copy that could be accessed with or without internet access which would show the most current data.
Does anyone have one they can recommend?