I'm down on the ground level in the park for the first time. I just talked to every body, and the doorman for the club says I must have a sponsor. At this point, even using walkthrough's, I can't figure out what to do!
Some walkthrough's say Mrs. Piermont will offer to sponsor me if I go up to her apartment, but she doesn't say that -- she said I was a stranger and won't talk to me anymore. Another walkthrough says I should use Reich's card on the shed door -- that doesn't do anything.
Any help?

You can use Reich's card on the shed door's lock. You have to pixel hunt for it but it's there.
On the Mrs. Piermont issue, have you tried actually going up to her apartment? Ring her up if you can and that should enable you to start the sponsor bit. And also, have you exhausted every possible topic with all the other people?? Sometimes that opens up other topic options for people you have previously spoken with.
Hope that helps.

I started speaking to Mrs Piedmont immediately after the doorman when she was milling around the pool. You can buzz her apartment but I'm assuming only two of the options appear if you haven't already told her who you are ("Pizza" and "Security"). You should probably wait and see if she comes back down. As for the shed door, yeah, I had problems with hat too, and as Judas said you need to look for the lock roughly on the centre of the door then use one of the cards on that. The item inside isn't needed until a bit later though.

Ok, I've opened up the shed door and gotten the shears. But Piermont still won't talk to me. I have no more dialogue options with the doorman, the guard, the kid, the gardener, or Mrs. Peirmont. She and her dog keep walking in circles around the pool and never go to the apartment. I can buzz the apartment but nothing happens (she's not there)...
getting frustrated..

Yes, Kotra. Thanks! I had forgotten to decrypt one of the files (the one that says Overmann is your father). So I wasn't able to tell that to Piermont.
Man, that was driving me nuts!

ineed that tuning fork i have gone on and continued to play without getting ,but, i know i need it for the crystal. can not get it

I've searched for spoilers and tips. I've tried everything I can think of. I've retrieved the secateurs. I've spoken to everyone by the pool until the only thing left is "see you later" or "never mind"...including Mrs. Piermont. I cannot get her to leave the pool area to go back to her apartment and I can't move along in the game until she does so. How the heck do you get the old bat to leave?

Thanks for the solution Kotra! (it really isn't explained in any walkthroughs probably because they expect you to follow the walkthrough word for word ;))

for anyone still unsure:
- if you haven't decrypted one of the files (the first) in Virtual Reality and looked at it afterwards via a LINC terminal, then you won't be invited by Mrs. Piermont. Thus, you have to return to the VR world (take the elevator in the security office at the highest level and go back into the interface via card to get into VR world) and there you can decrypt your documents in your inventory by using the decryptor on the documents. You will see the "?" disappear on the file. Afterwards you can look at a normal LINC terminal at the files (4 - Security Services). After seeing these files and talking with Mrs. Piermont again, she will invite you. And life goes on.