For this achievement, you need to gain a few affinity levels with all these characters...
Yui isn't a character you can use, but when gaining the affinity levels with some of the other characters, your gain scenes with Yui in and this is what counts for gaining levels with her. (You can check the scenes by going to your house and checking the TV and pushing
to go to the next screen)
This achievement unlocked for me after getting Agil to affinity level 2. (I already had the others at 4 having gone down the list in order). There is a separate achievement for getting all friends to affinity level 4 so just get everyone up to 4 and your obtain this achievement along the way. Make sure to always check to see if there is a green speech bubble after leveling a affinity level.
You can check your current rank with any of the character by pushing cn_start then go across to the player icon and click on friend list, you can then push cn_X and down the bottom on the right, it will say their rank.
The fastest ways to gain points with the people in party is by reviving them.
On the Extreme difficulty, the snipers one shot everyone. Using the Photon swords skill Sword Barrier you can swoop in and revive them.
On the Normal difficulty the way I've been grinding this out is by fighting the Honeycombed enemy in Solitary Sand (Large mountain to the right of the map). This will require luck and being good at dodging.
The alternative solution is take on quests and go farming monsters. This will also help to level up your characters.