When someone you are not connected with writes to you, the message arrives as a request first. This keeps you in control of who can reach you.
Open the Messages section. Pending requests appear at the top of your message list, separate from your ongoing conversations.
Click on a request and then Accept. The conversation moves to your regular inbox.
Click Decline. The sender is not notified that you declined.
Any registered insteady member can send you a message request. Messages from people you are not connected with always arrive as a request first, so you decide whether to reply. Once you are connected with someone, their messages go straight to your inbox.