Hi, do the login details that you have for the o2 website allow you to view your bill? To check, get logged in and click 'my o2' then 'balances and bills'. If you can't get into your bill then it's because the details you have aren't linked to your account so it won't let you buy the bolt on either, customer services will be able to give you a login that will work.
If you can get into your bill ok but still get an error with the family bolt on then...emm...I'm not sure!

Is the problem that you can't add it at all or is it with setting up the group?