Você deseja usar
GROUP_CONCAT()
Como SELECT p.id, group_concat(pcb.users_admin_id) as uid
FROM properties p
INNER JOIN prop_captured_by pcb
ON p.id = pcb.property_id
group by p.id;
GROUP_CONCAT()
Como SELECT p.id, group_concat(pcb.users_admin_id) as uid
FROM properties p
INNER JOIN prop_captured_by pcb
ON p.id = pcb.property_id
group by p.id;