Legal Question in Wills and Trusts in California

A power of attorney is your best bet. They can sign it and have it notarized at a U.S. Embassy or Consulate if they are U.S. Citizens or resident aliens, or it can be notarized, verified or authenticated, under the laws of the country they live in if they are nationals of that country. Alternatively they can just give you a written signed agency agreement, which would certainly empower you to find tenants, sign a lease for them, etc., but you might run into issues managing the property for them, if you have to deal with the government, banks, etc.