If you have multiple files to share across multiple Teams, the best solution would be to create a new Team, adding the appropriate Team members.
If you have the occasional file that you want to share with other Teams, this is doable, although not recommended as you will want to keep track of where the file is originally sourced from. This may change as Microsoft continues to update Teams.
Steps for Sharing a file across multiple Teams
1. Make note of the Team names that you want to share the file with (needed in a later step)
2. Go to the Team that has the original source file
3. Select the file (place a checkmark beside it) and select "Open in SharePoint"
4. Create a Share for the file. Either select the file by placing a checkmark beside it and select "Share" from the upper toolbar menu, or click the options icon (three dots …) to the right of the file name and select "Share" from the pop-up menu
5. Select "People in Brandon University with the link"; this brings up another menu, select "Specific People". By default the Teams or users this file is shared with are set to "Allow Editing", if you want it to be read-only uncheck the "Allow Editing" box. Click Apply
6. Type in the names of the Teams or users you would like to share the file with
7. Click "Copy link". A link box will appear, click "Copy"
8. Go to one of the Teams you would like to share the file with
9. On the toolbar towards the top of the screen, click the "+" tab to add a new tab
10. Select "Website" as the tab option
11. Enter in a Tab Name (could be the name of the file – keep it short) and then paste the copied link into the URL field (right click the mouse and select paste or press CTRL+V). If you would like the Team to be notified of a new tab, leave the "Post to the channel about this tab" checkmark selected and click "Save"
12. This creates a new tab at the top of the screen with the name you specified in the previous step. Instead of accessing the file through the "Files" tab, the file is accessed through this new tab. Changes made to the file will be reflected across multiple Teams. Note that once the tab has been added, users may have to click the button "Sharing Link Validation" to access the file for the first time in this Team
13. Repeat the steps to paste a link in a different Team
Steps to Remove Sharing a file across multiple Teams
1. Go to each Team that the file was shared with (each Team that has the 'Shared File Name' tab at the top of the screen). Right click on the tab and select "Remove"
2. Remove the share permissions from the original source file. Go to the Team the file was originally saved in
3. Select the file (place a checkmark beside it) and select "Open in SharePoint"
4. Open the details pane towards the top right-hand side of the tools menu bar
5. Click Manage Access, this will list the various shares for the file
6. Delete the shared link for the appropriate Team. Look under the section "Links giving access" to see the shared links to other Teams or users.
7. The original view does not show "who" the file is shared with, but you can click the down arrow beside the people icon and it will list the Teams or users the link is shared with
8. To remove a shared link, click the X to the right of the Team or user name
9. The file is no longer shared with the removed Teams or users