How do I manage permissions for external users?

Hello.

We have created a new site collection solely for the purpose of creating sites to share files with external users.  This works really well, we have enabled sharing, we share the site using external email addresses, they get the email message and then they can log in.  We share files and everything is great.

However, I'm struggling to find a way to manage the permissions for these external users.  When I go into site permissions, or library permissions, I can only see our internal users listed.  There is no sign of any external users.  Where are they?  How do I change their permissions, or remove them altogether?

If I log in to the Office 365 Admin I can see them listed under the users page, so they do exist in our tenant.

Thanks,

Dave