If anyone else should come across this, you need to RDP onto your TFS (on-premise) box. Open a command prompt. Change Directory (cd) to the Program Files folder where TFS was installed into. See my example below:
cd "C:\Program Files\Microsoft Team Foundation Server 11.0\Tools"
Within the directory above (path may be different for your TFS install), there should be a tfssecurity.exe file. You need to run that using the following command (within the same command prompt window you ran the above command):
tfssecurity /g+ "Project Collection Service Accounts" n:"{VISUAL STUDIO ONLINE ACCOUNT EMAIL ADDRESS}" /collection:"{FULL URL TO YOUR VS SUBSCRIPTION}"
Don't forget to replace the values within braces ({VALUE}) with the values applicable to your accounts!