Generally for a discount like that you have to call the dedicated number for that company. Sam's club is just advertising it and collecting a cut for each signup, you're signing up with Directv through a dedicated number.
Generally, retail offers like that cannot be combined with referrals, but would get the regular promotion available to anyone, like what is on the website.
The only extra discount beyond the national promotion I've heard of people getting combined with the referral offer is the AAA discount, and even that is difficult / rare.
Is the Sams Club promo better than the extra $100 off via referrals? You can see the regular national promo right on directv.com to compare.