Every time a user completes a client activity, I store the results and the user that completed the activity in a SharePoint list. The user that completes the action is saved to a Person field. This was working fine when activities were assigned to individual users. However, I've switched to use Roles that have a couple users, and when I store the results (via a smart object) I get the following error.

 An error occurred in the UATWorkflowCentral service instance. Server was unable to process request. ---> Error setting metadata fields - Fields do not match. ---> Group: GROUP NAME

Microsoft.SharePoint.SPException: Group cannot be found.

   at Microsoft.SharePoint.SPGroup.InitMember()

   at Microsoft.SharePoint.SPGroup..ctor(SPWeb web, SPSecurableObject scope, SPUser user, String GroupName, Objectb,] arrGroupsData, UInt32 index, Int32 iByParamId, SPGroupCollectionType groupCollectionType)

   at Microsoft.SharePoint.SPGroupCollection.get_Item(String name)

   at SourceCode.SharePoint.WebServices.K2SPFunctions.K2SPUsersGroupsFunctions.GetGroupId(SPObject spObject, String groupName)

With GROUP NAME being the name of my K2 role. 

Now, only one user actually completed the activity, so I'd like to figure out who exactly it was. I can't seem to find any way to extract that information.