SPFieldUserValue userValue = (assigned prior to this code section)
string userOrGroupName = null;
// userValue refers to a group
if (userValue.User == null)
userOrGroupName = userValue.LookupValue;
else // userValue refers to a user
userOrGroupName = userValue.User.LoginName;
SPWorkflowTaskProperties childTaskCreationProperties = new SPWorkflowTaskProperties ();
childTaskCreationProperties.AssignedTo = userOrGroupName;