Here are the basic "safe harbor" tax rules (not saying this as a lawyer):
No more than 15% of gross revenue (generally excludes initiation fees) can be from non-member sources. There are a bunch of IRS rules and regs about what constitutes a non-member source, which is why any member bringing out more than 7 guests at a time (assuming that is allowed) has to fill out forms stating basically that they are his guests and he isn't being reimbursed by any of the guests.
IF you blow by that number, the IRS may consider whether you qualify as a tax exempt social club. There can be extenuating circumstances. If you hold a big tournament, such as a US Open, that "occasional" exceeding of the 15% limit would not kill you (might be another story if you host an event yearly that brings in a lot of revenue, because then you really might be financing the tax exempt purpose with non-member income.
If you lose your tax exemption, you have to keep separate accounting for member vs. nonmember revenue and costs, depreciation etc. The upshot would be that you would be taxable on your income adjusted by the rules. It may be that all of your initiation fees would be subject to a 35% tax right off the bat (depending upon whether it's equity). Banquet business might be heavily taxed because it is very profitable, as is guest play.
Patrick, does the 400 member limit relate to whether you are a public accommodation, and so subject to all of those laws? AS far as I know, it has nothing to do with tax.
ON tee times, we have a big club, with pretty heavy play, and there is always a contingent wanting to go to tee times. Most of that push is for member play, not guest play. Our members who bring a lot of guests do NOT want to have to rush to the tee in the middle of lunch if someone is late. Much easier to just spend a few more minutes on the range or putting. We also saw a shift last year in the timing of guest play. In prior years, only 20% of our guest play was on weekends -- if it was equally distributed, you would expect to see 32 percent on weekends. Last year, weekend guest play went to 30%, a huge increase, which led to some significant crowding at times that used to be empty. Interesting.