Tello's July 4 promo, running now through July 6, is double Tello dollars when signing up with a referral. That makes 20 Tello Dollars, for both parties in the referral. Yo can use Tello Dollars to pay for additional months. (After your promo rate period is finished, if you're also taking advantage of a multi-month discount.)
This can be used in conjunction with the Stack Social offers (see https://www.nthcircle.com/forum/deals/1357-hot-deals?start=1210#52220 -- thanks, Isamorph!), or with the current Tello homepage offer (25% off, good for 6 months). The referral code is entered during the process of signing up for a Tello account.
A reminder on any of the Tello multi-month plans: if you change plans, or renew early, you'll lose the balance of the promo-- especially important with the Stack Social promo, since you're prepaying for that one. (You'd just lose future discounts with the Tello website offer.)
There's a selection of Tello Referrals in the Referral thread: https://www.nthcircle.com/forum/general/76-referral-links-allowed-in-this-thread-only-hungryghost?start=70 Scan through and find a member who's referral you'd like to use.