Yeah its possible, each character will have a tab for there own saves. You are correct on keeping track & there are a few ways people can join your game, bottom left during character creation, Escape menu will have a session button, and in the options menu there will be preferences for online play.
If you leave the game open to friends and someone joins & creates a custom character, that character won't leave your party ever. Presuming if there is a problem and someone joins, you could revert to a old save. Haven't tested that out personally though.
