To add members to a board, select "Add Members..." from the board sidebar. Search for a user by name or enter an email address to invite them to the board. Click their name to add them to the board.
You can add members to boards even if they are not already using Trello. Just enter the person's name and email address and click "Send." The user's permission level will be considered "Virtual" until they join Trello, but you can still add the user to cards.
Allow Org Members to Join
If your board is a part of an organization and you would like any of your organization's members to be able to join the board and collaborate, you can select "Allow Org Members to Join" from the board's settings in the menu. When you select this option a checkmark will appear next to "Allow Org Members to Join" and your organization's members can add themselves to the board.