Go to the email you received from SpareMusic and then click the invite link. Register and activate your account so that you're logged in.
You'll then see a purple banner across the top of your dashboard if you have an invite. Click "Review Invite" and you'll be able to accept and reject your split.
Here you can review the details of the release and see the share you've been offered.
Once you've reviewed the invite, simply accept or reject the invite using the buttons at the bottom of the page.
Once you've accepted / rejected the invite, you will not be able to view this page again!