One option is listed here:
Wife and I both use Blackberry Torches. We are constantly sending and accepting calendar requests. Using exclusively blackberrys, one sends a request and the other accepts...
Here's the rub: I just ordered a new iphone 5. Can't stand the blackberry anymore. Once I get it, how can we perform the same functions with the calendars? We both are currently using Outlook to sync our phones, but being able to instantly send/receive calendar additions may be an issue....is Google Calendar an option?