https://www.gamespot.com/articles/destiny-2-adding-a-new-way-to-get-double-xp-heres-/1100-6454841/

Bungie is rolling out a new way for Destiny 2 players to earn double XP. This is being done through what Bungie is calling a "Clarion Call." As part of this, beginning on November 17 and running through the weekend, players who fight alongside other clam members in their fireteam will earn double XP.

You have to be in a clan to get the double XP reward. If you're not already in one, all the information about how to create or join a clan can be found here on Bungie's website. It's a relatively simple process, and well worth doing to get that double XP reward. The double XP bonus begins at 10 AM PT on November 17 and ends the same time on November 20, Bungie announced in its newest blog post.