Cant share/attach files in MS Teams chat

NickPuzzle if they're added as a Guest, they'll need to switch tenants (but they won't need to sign out of Teams). 

If you go down the path of Shared Channels, it's a bit more effort as your Teams Admins will need to coordinate and set up B2B direct connct (B2B direct connect Microsoft Entra overview | Microsoft Learn) but it means they won't need to do any switching and your channels will appear in their regular teams/channels area. 

Here's some more info on Shared Channels: Shared channels in Microsoft Teams - Microsoft Teams | Microsoft Learn