Hi,

To add a new security group to an existing Odoo module without altering its source code, create a custom module that extends the existing access rights rather than replacing them. Define your new security group in a groups.xml file with noupdate="1" to prevent issues during module updates.

In your ir.model.access.csv file, add new access rights records for your security group, referencing the existing access rights records from the target module using their XML IDs. This ensures that your new group has the desired permissions without affecting the existing access rights. Optionally, you can adjust views to hide specific fields or buttons based on the user's group membership. Remember to declare a dependency on the target module in your custom module's manifest.

Reference:-

* https://www.cybrosys.com/blog/how-to-create-security-group-and-manage-access-rights-in-odoo-18

* https://www.youtube.com/watch?v=uUznwThpLmc

Hope it helps

Take a look at the Sales User (All Documents) group and you will see that it

1. inherits the Sales User (Own Documents Only) group
2. is inherited by the Sales Administrator group

This "chain" of three (or any number) of groups is what tells Odoo to display it as a dropdown.  Otherwise each individual group has to be selected.

So your new group needs to be added into that "chain"