The player.placeatme command spawns a fresh copy of any NPC, creature, or companion right next to you using its base ID.
Spawn a companion or NPC
Open the console with ~ (how to open the console), then run player.placeatme <base id>:
That spawns a copy of the companion. Find every follower's base ID on the companion IDs list. You can add a count to spawn several at once:
Spawning vs. moving the real NPC
placeatme creates a duplicate — useful for enemies or extra bodies, but for a unique character you usually want the original. To bring the real one to you, click them in the console to select their reference, or use their ref ID with moveto player.
Bring a dead NPC back
If the character you need was killed, select them in the console and use resurrect:
This revives the selected NPC where they fell — the safest fix when a quest-critical character has died.