In Spring '15 release, Salesforce introduce Manager Group sharing. This is very useful and utilize Manager field in the user detail, not by role hierarchy.
As admin, you can enable Manager Groups from Setup | Security Controls | Sharing Settings, look for Organization-Wide Defaults and click Edit button, scroll down to Other Settings and select Manager Groups and then click Save.
When Manager Groups sharing setting is enabled, user can manually share a record to Manager Group and Manager Subordinates Group by click Sharing button from any page layout, additional option Manager Groups and Manager Subordinates Groups will be available on top of current option.
How this sharing setting work? Let's use following role hierarchy for samples below:
1. Manager Group
Sharing includes a user’s direct and indirect managers, not the user himself.
Sample 1: a record owned by Andy, sharing the record to Managers Group: Peter will include: Steven and Jacky.
Sample 2: a record owned by Andy, sharing the record to Managers Group: Robert will include: Peter, Steven and Jacky, but not include Robert.
2. Manager Subordinates Group
Sharing includes includes a user and the user’s direct and indirect reports.
Sample 3: a record owned by Andy, sharing the record to Managers Subordinates Group: Peter will include: Andy, Robert, Danny, George and David.
Sample 4: a record owned by Andy, sharing the record to Managers Subordinates Group: Robert will include: Robert, Danny, George and David.
Reference: Sharing Records with Manager Groups