Contributors are your team members or translators assigned to translate one or several languages within your project. There are reference and contributable languages for each contributor. Reference languages are actually read-only, while contributable ones can be edited.
Lokalise allows concurrent editing, so if one contributor is working on a particlular translation, all other visualising the translation being edited will be notified that the translation is in progress.
Optionally, you can set someone as project owner granting same project access right as you. Only the project owners have access to project import/export as well as key editing and some other advanced features.
Proofreaders may mark translations as "proofread" further using this status when exporting the project or applying a filter in editor. Note, that you would need to enable "Proofreading" in project settings as it's turned off by default.
There is no limit on number of contributors you can add to the project.