Non-equity groups are not legal in the UK. The CAA allow up to 20 people to jointly own shares in a group, so the minimum value of a share is 5% of the aircraft. Shares do not have to be equal, but each member of a group must own at least 5% of the aeroplane.
If you do not own a share of the aeroplane (a non-equity group), you are effectively renting the aeroplane. For a rented aeroplane, the component times (eg. engine) must be within the manufacturer's stated 'Time Between Overhaul' period, which is usually 2400hrs and/or 12 years (dependent on engine and manufacturer). A 20% extension (2880 hrs or 14 years) is allowed provided certain conditions are met, eg. no metal in the filters etc..
If the same aeroplane is in group ownership (i.e. not rented) then the engine can carry on indefinitely 'on condition'. This would not apply in this case.
In summary, therefore, don't get involved in a non-equity group. If nothing else, you have no legal claim on the aeroplane and would not be able to claim anything back were it sold or disposed of.