United We Stand

United We Stand is one of the companion perks in Fallout 4.

Obtaining
To gain the perk, the Sole Survivor has to reach maximum affinity with Preston Garvey.

The quickest way to achieve maximum affinity is to head to a Weapons workbench. Make a save and attach a mod to any weapon. You do not need to build a new mod every time; attaching one from your inventory works fine. Make a save overtop of the previous save after Preston has "liked" this action. Now reload that save, attach another mod, and make another save over top of the previous one. This technique also works with Codsworth, so having both nearby while doing this process will help kill two birds with one stone. [This technique no longer works, due to the built-in cooldown period between companion "likes".]

Each action "liked" will add 15 points (7.5 points, as of game patch 1.5) to the required 1000 for reaching maximum affinity. Using this technique, with Preston starting at a base affinity of 0 it will required 67 weapon modifications to be attached (134 mods, as of game patch 1.5).

A second method involves having as many conversations as possible with him around. Go through Diamond City and do favors for people and resolve fights as a pacifist. After he likes you, go to the Third Rail in Goodneighbor and talk to Whitechapel Charlie. Buy a drink and play along until he asks you yes/no. If you say no, Preston will admire you.

Effects
The player character's Damage Resistance is increased by 20 points and the player character deals +20% damage when outnumbered.