To unlock the Strength in Numbers trophy in The Division 2, you will need to first recruit Grace Larson at the Theatre settlement.
To recruit her, you will need to complete main missions and side missions from NPCs at the Theatre settlement until she becomes available. Once she becomes available to recruit, you can talk to her on the top floor of the Theatre building.
Return to the White House and talk to Grace on the ground floor of the east wing. You will now have the option to create and join clans.
Both creating or joining an existing clan will unlock the Strength in Numbers trophy.