The left Brand & Links Column on your Brandboard will include a header for Collaborators and list you as the board owner. The header includes a +. Click that to view and manage current collaborators, and invite new collaborators. There are tabs at the top of the layer that will open, which you can use to toggle between management and inviting collaborators.
To invite collaborators, you'll first need to select a permission level. Collaborators can be View Only, or Collaborative, which means they can add their own Clipps to your Brandboard. You can invite anyone, whether they are a current Clipparoo user or not. Just enter a valid email address for each collaborator. (Or, if you've already collaborated with this person on another board, you can select them from your Existing Users list.) Next, you can preview the email that will go out, and then click the button to send your invitations.
Watch this video on inviting collaborators to you Brandboards.
Please note: This video will include references to "Client Pages" - this is an old name for Brandboards.