Guest editors are users you invite to view or edit your documents — free of charge and without adding them to your workspace. They can access and collaborate on shared documents but cannot create their own.
Workspace members, on the other hand, are paid members of your workspace. They can collaborate with you and also create and manage their own private documents.
To add a guest editor, see: How do I invite someone to view or edit a document?
To add a workspace member, see: How do I add a user to my workspace?