If you see people drinking alcohol on the street, you may call the San Francisco Police Department's Non-Emergency Dispatch line at (415) 553-0123. Drinking in public, including carrying alcohol in an open container, is illegal, period.
Remember: if you feel uncomfortable making a report, you always have the option to remain anonymous. When calling the police, provide a full description of the situation and note whether or not the problem is a recurring one. The violation is punishable with a $77 fine but, if applicable, the SFPD could take into account the fact that the trouble is ongoing.
However, if you see a person who is inebriated and in need of assistance, call the City's Mobile Assistance Program (MAP) at (415) 431-7400. MAP does not provide medical care, but can move the person to a stabilization center, if he or she volunteers to go. If the drunk person is violent and/or in need of emergency medical attention, you should call 911.
Be sure to cancel any dispatch if the person leaves before the police or other personnel arrive — and always remember to thank responders for a job well done.