Hi sknorth,
Once the external user accept the sharing site invitation
and enter with an account, the external user will be automatically added to the
Visitors group of your team site.
If you would like to grant editing permission to this customer,
you can change this group’s permission.
To set unique permissions, you need first stop inheriting
permissions from the parent. When permissions are no longer inherited, you can
remove any of the old permissions and add any new ones.
Here are step-by-step instructions:
1. Log in to the team site with the admin account. Click Library Permissions under Page panel.
2. On the next screen, in the head pane, click Stop Inheriting Permissions,
when a warning appears, click OK.
3. Choose Visitors
group, then click Edit User Permissions.
4. Then choose the permissions you want these users or
groups to have. If you want to make them be able to edit the website, you could
grant Contribute permission to this
group.
5. Click OK.
Thanks,
Ella Huang