Users, Members, and Guests are the three types of contacts you can work with on the Contacts page.
See "Managing Contacts" for more information about the Contacts page.
Users have a Workfront Proof login in your organization's account. If you are a Workfront Proof administrator, you can add a new user. See "Creating Users in Workfront Proof" for more information.
Members have their own Workfront Proof login in another organization's account (not your own). You cannot convert members to users in your account.
Guests do not have their own Workfront Proof login in your organization's account, but you have added their details to your account (for example, guest reviewers on proofs). You can convert guests to users. For more information, see "Creating Users in Workfront Proof."