Locating documents and sharing them with others within Microsoft Teams channels or chats
In my experience External members of a team site cannot collaborate on files stored within teams because teams stores documents on a SharePoint site which an external guest wont be able to access (you might be able to add them to the SharePoint site but I have not tested this).
To get around this I would create a One Drive Shared folder to collaborate with your external members and then have a link to the one drive folder within your Team site either via link in chat then Pin the message or by creating a shortcut in the files section.
Sorry for saying teams so much!!