Does my family need to go together for the 90-day immigration report in Chiang Mai, or can one member handle it alone?

Between 14 and 7 days before due date you can do your 90 days by post, you don’t have to send your passport just copies of main page, visa, arrival card and tm 30. Send them together with your last original 90 day report, a self addressed envelope and filled in tm47 form to:

Notify 90 Days

CHIANG MAI IMMIGRATION

Building A, G Floor

Promenada Resort Mall

192 - 193 Tambon Tasala,

Amphur Muang, Chiang Mai

Have done this several times, i use EMS to send and just a normal 3 Baht stamp on the self addressed envelope. It takes about a week before you receive the new passport slip.

If you still have time, you can do them by post. But you need to send 14 days before the due date.

Yes one person can do all of them.