When shared drives were created, the settings were to not allow access to outsiders of shared drive. The setting for new shared drives has been changed, but for some drives there is the chance that you won't be able to share to outside, making this article useless.
On the team drives page (drive.aegee.eu), go to your shared drive and click the small triangle next to the team drive name, then "Shared drive settings"
This is what one member of the speaker team sees on the team drives settings:
To fix the sharing settings, edit the second option to allow outsiders to get the files
Sometimes you simply must wait 5-10 minutes for the changes to be effective. Let's say 20-30 in the worst case.
If after re-trying to re-add and waiting a whole afternoon, the system still prevents you to share, hit us up at the helpdesk