If we have successfully shared a subsite on Sharepoint online(office 365) with an external user, using C#, Sharepoint.client.dll. He/She has accepted the invite and logged in to the Sharepoint subsite.
How do we assign him/her permission on some other subsite in the same Sharepoint instance, is it possible ?
Now that the user has accepted the invitation they will of been added to your directory as an external user, noted by #EXT#yourdomain. They will also have profile within that site collection.
You can invite/share other items in exactly the same way, the only difference is now they are part of your directory they wont need to link a Microsoft account again. If you don't want them received an invitation via email to notify them they have had this new content shared with them you can toggle this off.
The other way would be add the user to a SharePoint group and then added the SharePoint group to the permissions of items you wish to share. For tidiness I add all my user to SharePoint groups.