Since you're the group owner, you need a storage subscription that covers all files synced up to the group. You're currently at your file storage quota, so no additional files will sync. Currently, you'll see a warning next to the sync button in Zotero telling you this.

After that, they just need to join the group and sync. By default Zotero will sync all data and files.

Note that you also currently have your group set to allow file editing only by administrators, so non-admins won't be able to add files to the group.

BTW: I thought its free up to 500MB?

Oh, right. Thank you.

And one more question: Our company has a shared storage. Is it possible to connect it with the group library so its members will be able to download the files?

Or paying for the zotero storage is the only way to go?

Thanks!

If by shared storage, you mean a company file server or cloud server, no, not at this time. It's possible to set up and run your own version of the Zotero sync server, but that is fairly technically involved, and we can provide only limited support for that. Beyond that, Zotero storage is currently the only option for group library storage. Note that you could sign up for just one account for the company and have that organizational account be the owner of all of the company groups, so you would only need one storage subscription.