I am a financial advisor. I have friends who are residents of China that would like to invest in mutual funds, real estate, and annuities here in the United States. They are Chinese citizens - no social security number. Can this be done and is there a 30% backup withholding rate for any gains? Or is there a tax treaty in place with China that excludes or reduces that rate?

Profits from mutual funds or stock sales in not taxable in case of non-residents. They will file Form W-8BEN with the broker to report the non-resident status.

I recommend that they use Charles Schwab, who caters to international clients and whose website is available in Chinese.

As their financial advisor I suggest you first familiarize yourself with the US/China treaty;
http://www.irs.gov/pub/irs-trty/china.pdf
And the interpretation:
http://www.irs.gov/pub/irs-trty/chintech.pdf

That will acquaint you with the treaty benefits to your clients are entitled.

From the scope of the investment interests you describe they will need ITINs:
http://www.irs.gov/pub/irs-pdf/fw7.pdf

Real estate investments require special attention:
ITIN Guidance for Foreign Property Buyers/Sellers (http://www.irs.gov/individuals/article/0,,id=120219,00.html)