Add user to web subscription

[LTHOMASU]

I would like, when I create a user and after adding them to a group, to be able to attribute the rights USE, BROWSE, and READ to the group in question. Here's the equivalent in Command Manager: ("ALTER ACE FOR USER " $3 " GROUP "group" ACCESSRIGHTS CUSTOM GRANT BROWSE,READ,USE;").
The final objective is for all members of the group to be able to add the user to web subscription lists (via email).
The issue is that when I enter

Python

group.set_custom_permissions(
to_objects=user.id,
object_type=ObjectTypes.USER,
use='grant',
browse='grant',
read='grant'
)
I get the following error message: Error updating object with ID C82C... I-Server Error ERR004, Removal failed! No user with such permission exists!

Do you know why this isn't working and how I could allow each user in the group to be able to add the other members to a web subscription?

Thank you in advance!