If you ever wish to have a private conversation with another member, tap on their name, a dialogue screen appears, then tap the small box with "message" written in it to the right of their name. A new dialogue box opens. Additionally, the left column reads "In this Conversation" and the sender and addressee's name are listed. If you just begin typing in the big dialogue box on the first page, anything written there goes to the activity page for any member to read. My apologies if everyone knew this already!
Twojs&Guinness&OD