To be honest, I think it just depends on DBG's mood at the moment. I have 4 characters that are maxed out and for days/weeks they showed "capped". Now they are showing "sharing". But, my silver accounts that my maxed accounts are sharing with are still gaining in fellowship aaxp and regular xp, so Im not worried about it.
In my travels, I have found that if you are Max AA and EXP, you need to be the recipient of some fellowship experience for the system to turn you from 'sharing' to 'capped'. So, keep sharing on, remove yourself from the Exp group (where someone in that group is sharing with you), let them get exp, and when it cycles to give it to you, you should switch to capped.