In Sortd full screen mode you will notice a dropdown that says Personal Board. It's not possible to share your Personal Board (that's your private workspace). However if you click on that dropdown you will see an option to create a new board. Click on one of the new board options to create a shared board.
You will need to give the board a title and either choose an Organization that you've already created or create a new Organization (just a note that when you 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 Team Plan to use shared boards).
Once you have created your shared board and switched to it you will see an Invite Members link next to the board switcher. You can either click on that link to invite a team member or on the profile icon next to the Organization name in the dropdown.
That will 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.