Beware non-equity groups who's engines are 'on condition' would be my advice, incidentally.
I share your waryness, Smarthawk.
This is an area where the difference between 'renting' and 'private use' can easilly seem to be blurred.
Personally though, I think the dividing line is clear:
If you pay a monthly subscription to be a member of a group, and fly the aircraft as much as you like, without further charge, just topping up the fuel and oil to previous levels, this can be argued to be 'private use', and the engine can be 'on condition'.
If you pay any fee, in any form. with regard to individual flihghts, ie.
any form of hourly rate, then that is renting, and the engine cannot be 'on condition'.
MJ
Ps. I'm not involved with any 'non equity groups'