Does anyone know what happens to a group when ...

If the founder deactivates, say bye to the group. The other cofounders and other admins won't be able to do anything and the group will become useless.
If you leave the group first, it will ask you who the new founder should be! You should do it so the group remains useful if you're planning on leaving.
(I've left like two groups I was the founder of and got to do it myself so...)