I've tried and it's not possible.

After thinking a while I understood why. Let me explain my theory:

A belongs to categ1 categ2

B belongs to categ 2 categ 3

C belongs to categ1 categ 3

Okay if you list this three items:

• A
• B
• C

But if you try to group them by categ. You'll have to repeat A two times B two times and C two times. Group doesn't intend to repeat data it just group the data into sets without common data.

If group worked on many2many it'll do something like this.

• categ1: A C
• categ2: B
• categ2: (empty)

Because it'll show only once the registry and it'll catch them by priority.

Anyways this doesn't work. So if you want to group a many2many you have to do it by python code it's a little bit complicated but I think that there is a way to do it.