In Sortd you will notice that you start with a Personal Board plus any additional board that you created during the initial setup.  It's not possible to share your Personal Board (as that's your private workspace).  However any other board can be shared and you have an option to create any number of sharable boards.  

To create a new board, see the + button below.  

You will give the board a title and either choose an Organization that will own the board (if you choose to create a new Organization you will be asked to select a Plan - all the plans have a free trial but you will need to be on the premium plan to use shared boards).

Once you have created your shared board and switched to it you will see the round + Invite Members button on the top right of the board.

It will then show you a popup where you can invite someone by specifying their email address.

When you invite someone to a board they will receive an email invitation with a link to install Sortd (if they don't already have it) and instructions on how to accept your invitation.  On their side, they will need to navigate to the board switcher and paste the code into the Invite Code box.

They will then be able to see your board (under the Organization you created) in the dropdown.

Also see the following article:  How to I enable email sharing for my team?