Download the app and go under "My six flags" and then under "rewards and coupons". It usually says there when you have the "bring a friend for free" promotion.
Check Www.sixflags.com And you Pass number
It appears that you have to have an account with Six Flags. Per the site www.sixflags.com/magicmountain/plan-your-visit/season-pass-perks, you can show up anytime together and they'll let you in. To be safe, I'd recommend you reach out to SFMM to verify and register your pass.
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