I asked FCL at the CAA the exact same question. This was essentially their answer. If you have exceeded the 90 day rule, and want to log the flight with anyone else aboard, the only way you can do so is with an FI. You can however fly with another Group member who qualifies under the 90 day rule and who gives you permission to manipulate the controls. The other group member of course logs the flight P1 and is in command. You however regain your Group currency and confidence. Thereafter you go and fly the required 3 departures and landings.
Seems pretty silly to me, and doesn't in my opinion promote safety one bit, as the temptation is to go and do the solo bit after a 90 day lay off without the support of another current pilot.
The CAA are worried that the other current pilot might end up being a low timer coerced into the flight, and that is why you should find an instructor. I personally doubt that so far as groups are concerned, and wonder what evidence exists. I would be interested in the opinion of others.