After you have completed certain main missions for some characters you become friends with them. They will then call you on your phone every now and then and ask you if you want to hang out, or if you can do them a favour like pick them up somewhere and drive them to where they want to go. Helping your friends and spending time with them will increase your friendship level with them. Once a certain level of friendship is achieved you will unlock their special abilites. You can then call them on your phone and they will perform different favours for you.

Roman

Likes:
Bowling, Darts, Pool, Show, Drinking, Eating, Strip Club.

Unlock Friendship:
After the Roman mission Bleed out.

Unlock Special Ability:
At a 60% friendship level.

Special Aibility:
Free Taxi Service. A Taxi will pick you up and take you to a location of your choice, free of charge.

Little Jacob

Likes:
Darts, Pool, Strip Club, Show, Drinking, Eating.

Unlock Friendship:
After the Little Jacob mission Shadow.

Unlock Special Ability:
At a 60% friendship level.

Special Ability:
Gun Delivery Car. A car will show up from which you can buy guns.

Brucie

Likes:
Boating, Heliride, Show, Bowling, Strip Club, Drinking, Eating.

Unlock Friendship:
After the Brucie mission No. 1.

Unlock Special Ability:
At a 70% friendship level.

Special Ability:
Helicopter pickup. A helicopter will pick you up and take you to different set locations on the map.

Packie

Likes:
Bowling, Darts, Show, Pool, Strip Club, Drinking.

Unlock Friendship:
After the Packie mission Three Leaf Clover.

Unlock Special Ability:
At a 75% friendship level.

Special Ability:
Car Bomb Delivery. A car bomb will be delivered to you which you can set off from your phone.

Dwayne

Likes:
Show, Strip Club, Bowling, Drinking, Eating.

Unlock Friendship:
After the Playboy X mission The Holland Play.

Unlock Special Ability:
At a 60% friendship level.

Special Ability:
Gang Member Backup. A Car full of gang members will arrive and they will follow you and assist you.