I'm digging through my old workflows as I thought this is something I've done before....
In an SPD 2010 Workflow impersonation step, Add List Item Permissions, can't I feed it a semi-colon delimited list of user logins to assign permissions?
i've tried passing it
username1;username2;username3
,[email protected];[email protected];[email protected]
,domain\username1;domain\username2;domain\username3
but none works.