To add a client or external contributor to a workspace, go in any module of the workspace where you want to add them and click on "Add a participant" from the bottom left-side contributors' panel.
If your client or external contributor doesn't have a user account on your Wimi, you can directly create them a user account from the workspace.
The new user will only have access to the workspace in which they were added. Moreover, you can choose to restrain their access to some modules or folders.
This way, you completely control the data you wish to grant access to your client or external contributor.