If you're paying £120/month fixed (to cover hangarage, insurance, non hours-related maintenance etc..), and £70/hr wet, there's something not adding up there. The O-360 in a TB10 is going to burn between 35 and 40 litres per hour during standard low altitude bimbling, which at today's prices puts almost nothing aside for hours-related maintenance (50hr checks, oil, prop, engine etc..).
If you have a group whose policy is to split expenses as/when the arise, then fine. If not I'd be asking some questions. My plane has the same engine, and we charge £30/hr DRY. Pay for the fuel you use (courtesy of fuel-flow meter), at the current rate, and have the security of knowing that if anything blows up, there's money in the bank to pay for it.
There are good groups and bad groups, and grouping isn't always the best way to hour build. If you can find a good 'un, grab it with both hands, especially if it's non-equity and you're not contracted in for longer than you want. Just make sure you do your homework and run the numbers before you get involved in a group - if they're not run right they can leave you seriously out of pocket. Standard rule: if it seems to good to be true, it usually is.