I gave all existing users a value for the custom field:
users = User.where('NOT EXISTS(
SELECT 1
FROM user_custom_fields ucf
WHERE
ucf.user_id = users.id AND
ucf.name = ?
)', 'user_field_1')
users.each do |u|
u.custom_fields['user_field_1'] = 'invited'
u.save!
end