a little something about a few amateur pianists i know: they prepare a recital (sometimes it takes them years, depending on how ambitious the program is), they rent a hall and give a recital. usually, they don't charge the audience, so it's not really a business or something, but it's a recital. as for orchestras - the philharmonics will never play with an amateur, but there are independent orchestras (in most cities), and they will play with you (for a certain sum...). so, in one word: it's an expensive hobby, you must either have the money yourself, or find a sponsor (i have a friend who is a teacher - not piano teacher - and amateur pianist, she gives about 2 recitals a year, always for charity - she gets sponsors to pay the expenses, and she doesn't get paid for this, but she gets to play and to see some money going for a good cause).
one more thing about amateurs: i like going to their recitals, even if they don't have the technique or the knowledge to play like a pro, because they all seem to have a fantastic passion for piano, which makes up for everything else.
best luck