re: My intention is that my users only can change their own organisation. Is that possible?
yes, but only with the Link extension.
re: Right now when I do not choose the button “add” in “roles” it is still possible to add with that role. It makes no sense.
Make sure the user that your are testing with is of the correct role and that role has no capabilities.
re: And I can see Jetpack in the Panel.
Sorry, but I can not control what JetPack shows in the admin.
re: Is that going to look better if I buy Link?
Link will have no effect of what is displayed to your users in the admin. In addition to provided a method to “link” a Connections entry to a WP user it will add an admin profile page which is separate from the Connections admin pages that the user may add/edit their entry. This will not, however, disable or turn off any other admin pages that role has permission to view added by other plugins including Jetpack.
You might want to consider using Form with Link. Adding Form will enable front-end editing. You could then use another plugin to disable admin access to your user roles.
re: Can my users only see and change their own organisation with Link?
They will be able to see any entry they have permission to view, they will only be able to edit their own entry.
Hope that helps!