(1) Insert a cell
To insert a grid cell next to an existing cell, select two grid nodes of the existing cell (RMC on a node, hold the Shift key then LMC on the second node, the segment will be highlighted) then RMC to display options, select Line Mirror option as shown in Insert Grid. As a result, a cell will be added that shares a grid line with the existing cell as shown in Insert Grid
Figure 1 Define where to insert a cell.
Figure 2 A cell is added to the grid.
(2) Insert a cell
To insert multiple cells, select a grid segment (hold Shift + LMC), then RMC to select line mirror
Figure 1
Figure 2
Figure 3