By default the multiplayer mode uses weapon factories, when you run over them, it picks up that weapon, if you already have it, it adds ammo, Quake 3 Arena/Unreal Tournament style. However you can still implement the press E to pick it up mechanic.
Added by default now, no changes are required, just create child classes from Replicated_Weapon, this tutorial will be replaced with something else soon.
FPS Game Starter Kit 