Sounds like you want to use a combination of the property %sys.workitem.previousmanualsender% and possibly the Dynamic Actor role on manual activities.
sys.workitem.previousmanualsender will contain the login of the user who sent the last activity, so if you want to preserve it after a Manual Activity, you would need a set properties BIC immediately after the Manual Activity that takes the sys.workitem.previousmanualsender and stores it as a unique property; this is because certain automated BICs will overwrite this system property with the name of the BIC.
Once you have that stored as a unique property, you could leverage the Dynamic Actor role to then only have a subsequent Manual Activity contain a work item for that specific user as opposed to the group.
Here is a Precisely Tech Talk that explains Dynamic Actor and how to implement it: Link
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Matthew Hounslow
Amplifi
Dallas TX
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