It usually works one of two ways

Co-op: Players all create a new character (or strip their main naked) and play through the game together, do pretty much everything together and all progressing at the same rate. Most players go this route. You can do a lot when you work with a group, but you'll need a lot of resources to equip everyone.

Mercenary: You build up and gather resources in your singleplayer world, then take your best gear and head into someone else's world to help them out with some tough content, or just to hang out. When you've helped them do whatever they needed help with, maybe they give you a little somethin' for your trouble. Or alternately you go through the content solo and invite a mercenary to your world for boss fights or ore runs.