I don't know how I can share assets across workspaces.
I found that we can share a folder, when I click right on folder, I don't have any feature to make it shareable. screen shot below
"Global Emails" may be a system folder, you will need to create a new folder. Another possible reason is that there needs to be assets in the folder in order to share. You can move the assets to the folder and then share the folder.
You will need to create a folder for each component.
For example, you will need to create a new folder under Template folder, right click and then the Shared folder option will be available then.
Also, you can only share the following:
As Stephanie mentioned, you can only share some assets - not all. Vote these ideas up:
Global assets for workspaces
Allow forms (and emails/LPs) in Design Studio to be shareable across workspaces
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