How to implement many-to-many relationship with one table
You cannot do this with one additional column You need to create a new table for relations between employees. 

CREATE TABLE IF NOT EXISTS emp_relations(
emp_id1 INT UNSIGNED NOT NULL,
emp_id2 INT UNSIGNED NOT NULL,

FOREIGN KEY (emp_id1 )
      REFERENCES emp_list(id),
FOREIGN KEY (emp_id2 )
      REFERENCES emp_list(id),

UNIQUE KEY(emp_id1 ,emp_id2)  
 );


Link both columns to emp_list.id. Make emp_id1 and emp_id2 together unique.

This way you will have hierarchy and many to many relations.

Mysql - Foreign keys

Mysql - Unique keys