How do I do this?

[Konsu-kun]

When In dungeons, how do I summon people to help me in combat?

[Falbelmz]

Hey, welcome to the forums! The people you are referring to are NPCs, I presume? If so, these are the actions you need to take:
1. Press Alt/Esc to open the menu
2. Select Story
3. From the drop-down options, select Partners
4. Use the left partner choices to choose the NPC you wish to summon
5. On the top right hand side is the "Summon" button. Select that to summon de partner! (Note: Press dismiss to remove the partner from service)

[Funaho]

Also, once in a while your summoned companion can get glitchy, and they just start fighting thin air and are not much help to you unless they manage to accidentally hit an enemy while they flail around randomly.
However, you can dismiss them and then re-summon them right away to reset their behavior.