What you get told about
Assignment, mentions, comments on work you are involved in, and changes to tasks you are carrying. Notifications are always the result of somebody doing something, and you are never notified about your own actions.
A failed notification never means a failed action. Notifying happens after the change itself, so the work lands even if the message does not.
Two feeds, one bell
The bell merges notifications with pending workspace invitations. They are different things: a notification is addressed to an account, while an invitation is addressed to an email address that may not have an account yet. Merging them is what makes the bell the single place to look.
Marking something read is yours alone. Nobody else can see, read or clear your inbox.
There is no Share button
Everything in a workspace is already visible to everyone in that workspace, and your inbox is private by definition. A Share control would only ever promise something the product does not do, so there is not one.
To bring somebody into a piece of work, assign them, add them to the project, or invite them to the workspace.