Apple Logic Pro X - Manual (Page 670)

Chapter 20 View and edit music notation 670
Add and edit barres
A barre shows where the performer places his or her finger over several strings at the same fret.
You can add, edit, and delete barres on a chord grid.
Add a barre
Do one of the following:
m Drag the left or right edge of an existing dot to the right or left.
m Place the pointer over a string, then drag to the left or right.
Move a barre
m Drag the barre to a new fret position.