Guests can initiate Conversations with Platinum and Diamond Club members, if they match what those members are seeking, AND if they have not already received two messages from other Guests that day. To contact other types of members, including other Guests, a Guest user must upgrade to Premium at least, or if using the App, choose the Plus add-on.
Guests can however reply to any message sent to them but they only have seven days from the date of receiving the message to reply. After then, Guests will need to upgrade to reply to older messages.