If you have not heard of JoinFS, it is a software utility that runs separately from Flight Simulator that makes it very simple to connect with others for multiplayer online flying. It requires very little setup, and figures out most of the important details for you. It also comes with additional features such as a flight recorder that allows for creating self-recorded formation flights.
It is easy to download and install, and well worth checking out.
You may find it at:
The main stable version of the software can be found under their "Install" link on the main page. However, if you would like to keep up with the bug fixes and various feature improvements that the author is working on, you may register in the JoinFS forums, and under the "News" forum, the latest test releases are also available. You can have the "TEST" version and the main version installed at the same time.