You can click the 'Invite/Share' (people icon) button on the gray sidebar which will give you a link to your board and you can copy and send to those you want to add.
Once this has been done, the user should log into Waffle. After they have logged in, the board will be added to their 'Your Boards' list.
Please note: the collaborator must have appropriate access (Write access) within GitHub to work on the project. It is not possible to update this access within Waffle - it must be done from github.com.