People experiencing homelessness are eligible for the stimulus payments. You do not have to have an address or have filed a tax return.
This IRS link will provide you with information so that you can assist people in getting their stimulus check. https://www.irs.gov/coronavirus/economic-impact-payment-information-center
Attached you will find the IRS Questions and Answers (FAQ) that provides a wealth of information.
https://letsendhomelessness.org/wp-content/uploads/formidable/31/0326_001.pdf