I took a few screenshots for those who prefer them. Below is the location of two drunk men in the middle of the city at La Puerta/Palomino Avenue.
You must be playing as
Trevor to be able to deliver people to the Cult.
Once you get there, the man in the grey top will ask you to give him a lift in his Land Rover parked behind you. Get in and instead, take him to the new marker on your map all the way to the top of the map labelled "Altruist Cult". The achievement will unlock after the cutscene.
Get ready to drive up some mountains!
I found another one today, but this one seems to be a little bit more unreliable so don't depend on it always being in the set location. The event has a women getting kidnapped by the Lost gang as they take her into their van. You should approach with caution and kill the gang members before rescuing the women.
I found this same event twice in the same position as shown on my position on the screens below. The first time it was 4:00am but failed the event, the second time I found her at 19:00pm.
Note: When driving the women to either location, some gang members will come and attack you on bikes so be cautious of that and take care of them asap.
The location of the Altruist Cult:
Hope these help.