Shares can be a bitch to re-sell and any self respecting group is going to grill you as to your motive for joining. Depending on the type and make up of the group, short term hour builders aren't always going to be welcomed with open arms!
Finding the right and a receptive group for you isn't necessarily going to be that easy and you are opening yourself up to the risk of a shared cost. Extreme example but the engine goes bang and they dont have enough in their engine fund to cover it. Time for the group to dip into their pockets!
It's worth considering but it's not as easy as you perhaps imagine and may just be easier to negotiate some block time at a discount with a club or even a group. A group with low hour utilisation may be very happy to sell some hours to bolster their coffers.