How do you group things and make sure it doesn't fall apart?

I made a plane and then grouped it. After that, I tested and found that it completely fell apart

If you are making a non anchored group, make sure to Weld them together or via Motor6D.

I recommend using this plugin: RidEdit Lite

If you are making an anchored group, then just... anchor them... HUH

Click Your group then, go to the top of your screen and press anchored. It should work.