Tryin to help a Buddy out with this but not finding much good info. I realize that according to MS to accomplish Outlook Calendar sharing, the systems must be on MS Exchange. These systems are not on Exchange Servers. ISP email only.

I have found several pay apps that reportedly let non-exchange email accts share the Outlook calendar but I was hoping to find something free.

All they really want to do is to share their calendars with each other, no folder sharing or anything else. Does anyone know of or can recommend a stable, free app to accomplish this? It's for a very small business and must be reliable.