Thanks for the notice, I have added a warning in the README of the repository.
@eivind I’m quite surprised by your questions. Multimedia directories have been integrated with Nextcloud for YunoHost for 5 years now. Is your app older than that? (upgrades may have missed adding the directories).
As you can see with the following command, each user has a Multimedia Share directory that’s common to the whole server.
ls /home/titus/Multimedia/Share -la
lrwxrwxrwx 1 titus root 31 Jul 31 16:37 /home/titus/Multimedia/Share -> /home/yunohost.multimedia/share
You missed the ACLs (+ signs):
$ ls -la /home/yunohost.multimedia
total 20
drwxrwxr-x+ 4 root root 4096 Aug 24 15:08 .
drwxr-xr-x 13 root root 4096 Aug 24 15:03 ..
drwxrwxr-x+ 6 root root 4096 Oct 1 12:12 share
drwxrwxr-x+ 6 titus root 4096 Jul 31 16:37 titus
-rwxrwxr-x+ 1 root root 2514 Mar 13 2021 ynh_media_build.sh
$ getfacl /home/yunohost.multimedia/share
getfacl: Removing leading '/' from absolute path names
# file: home/yunohost.multimedia/share
# owner: root
# group: root
user::rwx
group::rwx
group:multimedia:rwx
mask::rwx
other::r-x
default:user::rwx
default:group::rwx
default:group:multimedia:rwx
default:mask::rwx
default:other::r-x
The ACLs do give write access to users belonging to the multimedia group. By design, only app users such as nextcloud or any other app using the Multimedia directories feature belong to it. So, if you add files to the directories through Nextcloud, it should work.
If you want your human user (like my titus) to have write access, indeed you will need to do sudo usermod -a -G multimedia elvind.
Note that the Multimedia feature is not enabled by default, it’s up to each app packager to decide if it’s worth integrating.
(erf, @wbk was faster, though their solution was a bit more technical than a simple usermod 
)