When they state "Over 1 MILLION XP", does that mean you will be instantly rewarded with a lot of XP once you upgrade to members? Or are they referring to new quests and other means to achieve more XP?
As the Sizzling Summer FAQ says,
How much XP is there for each month of the Sizzling Summer promotion?
Stay a member throughout August and you’ll get 350,000 XP to use. Stay a member throughout September as well, and you’ll get another 700,000 XP on top of that. That’s over 1 million XP in total.
Those two bonuses are combine to make the "Over 1 Million XP" reward that you refer to. The FAQ also says that this XP will be delivered by a promissory note:
When and where will I get the XP Promissory Note item?
It will be placed directly into your inventory at the end of each calendar month, unless your inventory is full. Then, you can collect it from Diango at your leisure.
So, the XP is a direct reward for membership, with almost no additional action required on the part of the user.