What you can do: Contact the Shelter Hotline and request transportation. Do this by emailing firstname.lastname@example.org or calling (202) 399-7093. Provide the time you saw the person/people, the location, and a description (male/female, clothing).
You can also let a person who is homeless use your mobile phone to call the shelter hotline. They should call (202) 399-7093. If they don't want to use your phone, have them dial 1-800-535-7252 from a pay phone.
Since it's wet and will be so for the next couple of days, consider buying adult-size socks, hats, gloves, or mittens and donate them to UPO. (Details about making a donation) UPO operates the Shelter Hotline (hotline and the van) and they are in the best position to distribute these items.
Finally, program your mobile phone with the Shelter Hotline email, email@example.com, and phone number, (202) 399-7093.