Use the built-in chat to communicate with members.
Click "Messages" in the sidebar.
Click "New Message" and select the member. Or click an existing conversation to continue.
If you are already looking at somebody — Members → click them — "Message" is in the row of buttons under their name, and opens the conversation with them already selected. It works whether or not you have ever messaged them before.
Type your message and press Enter or click Send.