There are indeed two ways of meeting your objectives, if your colleagues staying airside is not negotiable.

You can simply pass through flight connections, have your lounge meeting. Then when you are ready to leave, go down to the customer service desk towards gate 13 and request to go landside. You don't have to give a reason (however do NOT say you need a smoke), and you will be offloaded from Ready To Fly, and your checked baggage either returned to you or sent to a separate part of the internal warehouse. You will then be escorted through a gate down to the arrivals level, you go through passports. The wait time for the HAL escort can be up to 59 minutes, but I gather they often let people through more quickly.

You can then go into town, come back to T5. Ideally you then rescan your boarding pass at the entry gates, Ready To Fly is set up again for you and hopefully your bags are reallocated to your flight. Sometimes, though, you will need to pass to Visa Check Desk briefly, since it sometimes falls over.

Unofficially you can just take the transit to T5C from the lounge, stay on the transit at T5C, ignore the recorded messages to get off, and return to T5A, you are then in the arrivals stream anyway.

When you return you should allow a good 20 minutes to get your Ready To Fly record back in sync, since it will still think you are airside. You may also lose your seat assignment, since the easiest way to sync records is to offload you and reload you. You will not be able to go through the entry barrier in Departures until you have done this, it will not let you through since it thinks you are still airside.

I'm not sure I would recommend either course of action, unless you were a T5 regular, but that is your option. As you can probably tell, it's somewhat easier if you are travelling without checked luggage.