Note that you cannot date a girl when you are on a mission.
In order to unlock a friend's special ability, you must have a high 'like stat' for that friend, and this stat is based on how regularly you hang out with your friend.
Call your friend and perform activities with him on a regular basis to increase your 'like stat'.
This stat (amongst other friend stats) can be accessed from the pause menu under the 'Stats' section.
Friends and girlfriends become available as you progress through the storyline.
There are five friends and five girlfriends in total, and each character has a unique set of preferences.
Each friend (and three of the girlfriends) provide special unlockable abilities in return for maintaining good relationships with them.
There are five friends in total, which become available at different times during the storyline.
Don't take your friend to an activity that he doesn't like or your 'like stat' will reduce.
You can perform upto four activities on each outing in order to give your 'like stat' the highest possible boost.
If your friend calls but you aren't able to hang out, respond positively and then call immediatley afterwards to cancel the activity. There are five different girls which Niko can date, two of which are unlocked automatically during the storyline, and the rest of which are available via the in-game internet.
Call one of the girls to arrange a date, then pick her up within 1 hour (2 real-time minutes) and take her to one of the activities shown on the map.