According to this support page, by mentioning a friend in a checkin shout, you will cause that friend to be checked-in with you. However, when I POST to /v2/checkins/add and include well-formed 'shout' and 'mentions' parameters, it only tells my friends that I mentioned them in my check-in, as opposed to triggering a delegated check-in (like the native mobile apps are capable of doing).
Querying /v2/checkins/CHECKIN_ID reveals that the checkins created by the API are completely missing the 'with' field, while it is present in checkins created by the official apps.
How can I populate the 'with' field via the API?