Grab is currently offering $3 off for new users who sign up under a referral. The referrer will also receive a reward for each successful referral - presumably also $3 off their next ride.
You can sign up under a referral link using the 'random' link in the grey referral box below.
If you'd like your referral link to also be included in this random pool, login to CheapCheapLah and come back to this page where you'll notice an "Edit" button to add yours.