Chase periodically increases the signup offer on their United Mileage Plus card to 50,000 miles plus 5,000 additional miles for adding an authorized user. That offer has returned (along with a $3K spend requirement). However, Doctor of Credit reports availability of a slightly better offer. This one includes a $50 statement credit after first purchase.
Getting the better offer is very easy. Simply search for any flight on United.com and you should see this:
Interestingly, if I copy the URL for the above offer and paste into another browser, the $50 statement credit disappears so there’s no point in sharing that URL here.
Both offers can be found under Chase… Airline Cards on my Best Offers Page. I listed both since the ability to find the $50 statement credit offer could disappear at any time.