There are 3 steps you need to do before an external user can add their data to your vault:
Invite the user through People & Teams --> External users. When you invite an external user, he or she is granted the lowest permissions possible (External viewer)
Once the customer accepted your invitation, you can change the permission level. Once again, go to the external user and select the name of your client. In the sidebar, you'll see a pencil next to the name. Click on it to edit the user permissions and set it to 'external member'.
Next up, go to your vault and select the folder you'd like to share with your client. Select 'manage' on the right side, and then 'external access'. Type the name of your client and click on 'share'.
This will grant an external user the possibility to add their data into a specific folder in your vault.
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