How do I send a private message to a fellow Flutherer?

Click on the link under their avatar or search for them in the search box to get to their profile. You can leave a comment for them in the “Comment” box. The default is “marked as private’. If you don’t want it to be private, uncheck that box.

Or after fluther.com you can type in /users/<member’s account name>.
Example: www.fluther.com/users/AstroChuck
Then just scroll down to where it says *Leave a comment” and do just that.

What my kids said ^^ plus:

If you’d like to alert them to it, or even better, ask them first in the thread before jumping into their private space: we call it PM and PMing (for Private or Personal Message).