If someone is still trying to solve this, you need to use Apps> Google Workspace> Gmail>Confomity, not Apps> Google Workspace> Gmail> Routing.

Go to "Content conformity" or whatever this is called in english. Clic add a rule.

Assuming your group address is [email protected]:

Step 1: Select "Outgoing" and "Internal - send"

Step 2: select "ALL of the following match the message"

a) Clic "Add" → "Advanced content match" → Position "Sender header" → "Matches regex" and add the following regular expression: (?i)group@domain\.com

Validate it, and then

b) "Add" → "Advanced content match" → Position "Recipients header" → "Not matches regex" and add the following regular expression: (?i)group@domain\.com

The (?i) part is probably not mandatory but it does not hurt.

The b) test can probably also be skipped, since adding [email protected] as a recipient if the address is already within the recipients list won't change anything.

Step 3: "Also deliver to": [email protected]

Save.