I can't say what their current policy is regarding membership, but regarding the "best girls" thing, they're probably just booked out if you show up during weekends.
Another noteworthy thing, if you somehow get to meet a girl you like, ask for her LINE (or Twitter) and book directly with her next time. It'll set you back an extra 3000 yen at Y-shirt with their new system, but it's a small price to pay for getting to bang someone you know for sure you will enjoy banging. Cityheaven is also a good resource, as girls sometimes post their LINE or Twitter account on there as well. Calling the shop directly is probably a little bit more risky these days, considering DH used to let foreigners book by calling, but now Y-shirt apparently doesn't do that anymore. There's a note on their Cityheaven page that says "English-sounding nickname will not be allowed to make booking". I'm always gonna stick to my alias, book directly with the girl whenever possible, and the old dudes can go fck themselves lol