Hi there, you can work with WF to help clean this up on the back end, but here is info on what causes this to happen:
https://experienceleague.adobe.com/docs/workfront/using/administration-and-setup/manage-wf/configure...
From @BrownPaperBag in a similar thread last year with this from support:
"Member accounts are created when the user is active with another company and is added to a different company's workflow. Guest accounts are created when the user does not exist at all in ProofHQ, and they are added to a workflow, or if the user does exist in ProofHQ, but is deactivated on the other account; if they are deactivated, the system does not add them as a member since they do not have an active login to another account, but instead will create them as a guest user.
When a user with a matching email exists, in their own or in another proofing environment, Workfront creates an alias email address by adding the user’s account id as a suffix to their email. For example, username+accountid@domain.com. Users will still receive proof notifications in the event an alias email is created." - this last bit is reiterated in the link I posted above.
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If this helped you, please mark correct to help others : )