Volumes can be written about this. As a starting point, maybe have a look in the pfa website, it may have info on this subject, also the
www.flyer.co.uk forum on group ownership.
As a kwikky starter though:
Pros (over hiring a club a/c):
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Costs more manageable
aircraft more available
experience sharing with other group members (social aspect)
experience ownership without huge outlay
Get to "own" an a/c you otherwise couldnt - eg complex types
availability = you can get to other places!
Cons (compared to outright ownership)
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May have to dip into pockets for lump sums when big things go wrong (like engines, props)
May be "bad eggs" in the group - ie non- or late payers
Aircraft may be just as booked up as club aircraft
Need to get a consensus to do improvements or mods (eg new paint job, kit upgrades, etc).
Shared liability
I'm sure others can add to/disagree with/discuss these points...