Your friends should have a way of communicating which can be installed on their Macs. For example, the most common program Skype. Skype is multiplatform and it can be easily set up. When you open an account here, you are able to call your friends and send them messages over the Internet.
Download version for Mac computer: http://www.skype.com/en/download-skype/skype-for-mac/
You can also create Group Conferences where many users can be added to a session. It's the easiest and simplest way to chat with friends from another country.