I would suggest asking for the accounts going back 3-5 years. Look and see what the bills have been like
Then talk to their mechanic, bear in mind he may be reluctant to say anything bad about a customers aircraft in case it gets back to them. Read between the lines of his answers. You may want to get an independent mechanic to inspect it.
Go look at the aircraft when it is in for its annual and all the cowls and covers are off. If it looks good then it is a good sign. If it looks a mess then you should be concerned.
You can make estimates of when a new engine etc. will be required and therefore the cost implication.
See if they have an engine fund (we do). We put £10 per hour into it, a new member gets/buys there share of this fund.
Meet all the members of the group and ask whether you like/trust them enough to go into a financial arrangement with them.
Do they always fix problems immediately and properly or are there quite a few things that are being put off.
In my experience having a lot of rules doesn't help as there are always too many grey areas. For example supposing someone damages the aircraft this is obviously their fault. But what if they claim it was caused by something wrong with the aircraft? Who then pays. Its best to be on good terms with he other members rather than pulling out the rulebook.
Generally owning a share is lower flying cost than renting because private C of A is cheaper than a public one, there is no profit element and you can do things yourself like cleaning etc.. There is also pride of ownership and the ability to have the aircraft for a whole day or weekend (or maybe a week) without minimum hours. But check the group rules on this.
I have owned a share in the same group aircraft for 15 years and it has worked very well with good availability.
If you find something you fancy then start asking lots of questions. It is buyer beware but if you use common sense and take your time then I'm sure you'll not regret it.
Good luck