If there’s a sitter you want to message, you can do so by selecting Invite sitter on their profile. Once you’ve done this you can message them whenever you need to.
To reply to messages you’ve received from other members go to Conversations on the bottom toolbar.
Along the top there are the different categories, such as Confirmed sits and Inbox. Select Inbox and you’ll see all your messages. Select the message you want to reply to. You can then write your reply and send it to them.