To do additional grouping in a solution, follow the below instructions.
Click on the flow/s you want to do additional grouping for. Click on Flow Details as highlighted in the below image.
The below page will appear. type in the name you want to group the flows/ buffers with and add a right arrow ( > ) before the flow title. Click save to save the new title.
Illustrated in the image below is the grouping of the flow in the solution.
To add a buffer to the addition group in the solution, click on the 3 dots circled in red next to the buffer name and click on Edit as underlined in the below image.
The below page will appear. Type in the exact grouping name you used to group the previous flow and add the right arrow ( > ) before the buffer title.
Illustrated in the below image are flows and buffers grouped in a solution.