Make Dome Seating

How do I make circle on the second floor? For some reason, I’m not able to draw a line on the top of the dome to find the centre point.
I want to make a circle around the square on the second floor to represent somewhere people can sit with rounded edges and thickness, on all 3 sides of the elevator except the front side.

51H_SOUTH_Simplified18.skp (5.9 MB)

I also want to group the top part of the elevator. How do I add faces to a group that has already been created?

Double click a group to enter its context.

Try using the tricks I told you in a previous answer to a previous post:

Create a new Tag, call it Dome or whatever suits you.

Assign this Tag to your dome group and then hide this tag. The dome will be masques, opening easy access to the model.

Make the floors and elevator shaft another group to protect their geometries from upcoming new geometry.

Trace the circle anywhere on the second floor.

Add 2 radiuses at 90° then delete the quarter of the circumference delimited by these radiuses.

Select this circle and place it where you want it.

Cut it.

Open the group containing the second floor.

Use Paste in Place. The circle will be inserted in this group and at its previous position.

Unhide the Dome Tag, if needed.