The following article refers to functionality in Workflow Builder, which can be found by going to:

Administration > HR > Workflows

This option allows you to trigger workflows against employees with a manager level job role only.  This can be used to restrict a workflow so that it only triggers against such employees. 

When building or editing a workflow, the option can be found towards the bottom of the screen, labelled 'Only manager roles?'

PLEASE NOTE -

This does not relate to an employee's user group, only their job role, so it would not trigger against a user with a Manager user group unless they also had a manager-level job role.

Further Resources

For more information on job roles click here.  Training videos on the topic of building workflows in general can be found here.