There are different ways to share with Taskfeed
To share a particular Taskfeed Board with another Taskfeed user:
Add them as a Board Team Member.
The Board Team Member can be any Salesforce user in your Org who has a Taskfeed License.
To share Boards automatically with another Taskfeed user, manager, or a group of users:
Ask your Salesforce Admin to create a Taskfeed Board Sharing Rule.
To share a Taskfeed Board with someone who doesn't have a Taskfeed license or outside of Salesforce:
Use External Sharing.
When you share a Board using External Sharing, viewers will have READ ONLY access to the board.
You can optionally mark certain tasks as Internal Only if you don't wish external viewers to see them.