Samba share guest readable but group writable

I am trying create a samba share which is readable by anyone (guest/anonymous access) but only writeable by a particular system group. However it is not writeable to anyone.

I have created a new directory and set permissions as follows:

sudo mkdir /mnt/testshare
sudo chgrp testgroup /mnt/testshare
sudo chmod g+s /mnt/testshare
sudo chmod g+wrx /mnt/testshare

After which that directory is listed as:

drwxrwsr-x 3 root testgroup 4096 Apr 24 07:05 testshare

I have created an entry in smb.conf:

[test]
    path = /mnt/testshare
    write list = @testgroup
    create mask = 0660
    force directory mode = 02770
    guest ok = Yes

And restarted my samba server.

After doing this I am able to read all the files in that directory as a guest or when I'm logged in, but I'm unable to delete or modify files even when I am logged in as a user in that group (testgroup).

I have checked that I am able to create files in that directory as the same user by ssh'ing in, so the system permissions are ok.

My question

How should I configure samba to allow this?

Output of smbd --version is Version 4.2.10-Debian