Using the "wall".
This forum includes a few methods for communicating with other members.
The first (obviously) is via the discussions. The second (equally obvioulsy) is via PMs.
These work as you'd expect in terms of who can see your posts (everyone, and the addressees only, respectively).
There is a third way, via your activity "wall". This is where you can broadcast what you're up to across the whole forum membership. Any member can look at the activity stream via the "Activity" option. You can inform specific people about an activity by including them in the update with the "@" preface but this will still be visible to all members.
You can write on your wall from your profile page. Immediately underneath your frets score is a text box titled "activity". Anything that you enter there will display on your wall (immediately underneath the text box) which is also visible to anyone else looking at your profile page. It will also display in the general feed of recent activity that is visible to everyone via the "Actvity" option.
Note that, even if you use the "@" preface to direct your update to a specific individual member, your update will still be shown in the general activity update which is visible to everyone - so use your wall carefully and don't include any detail in updates that you don't want to be visible to all members.