After creating a curtain grid, you can place mullions on grid lines.
Add a curtain grid to a curtain wall.
Click Architecture tabBuild panel (Mullion).
Select the desired mullion type in the Type Selector.
On the Modify | Place Mullion tabPlacement tab, select one of the following tools:
Grid Line: When you click a grid line in the drawing area, this tool places a mullion across the entire grid line.
Grid Line Segment: When you click a grid line in the drawing area, this tool places a mullion on the individual segment of the grid line that you click.
All Grid Lines: When you click any grid line in the drawing area, this tool places mullions on all grid lines.
Click in the drawing area to place mullions on grid lines as desired.
Click Modify.
Mullions resize to the grid line, and automatically split at an intersection with another mullion. You can change the properties of mullions.