Safest way to add a recipient is to click on their avatar (or their name next to a post they'd made), then click the "PM" button on the next screen displayed.
The PM button under a post is broken and doesn't add a recipient.
To reply to a received PM, click on the "Reply" or "Quote" button under the PM as appropriate to begin writing a response.
The recipient is then automatically (and last I checked, correctly) added.
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Giraffe: a ruminant with a view.