This should be both abstract and specific. Gaining or losing members can be presented as a problem of scale, in which case it should be abstract, but always followed up with a personal example. Remember your principle to make everyone human.
To make the gang human, you don't need to name and act them all out, but you do need to put a human face on the gang any time you "handle" the gang mechanically. You should never be talking about the paperwork of gang sizes except as a side note to the action. It's not enough to just say "okay, you've got more followers, the gang representing them increases by one step." Instead of discussing paperwork and bookkeeping, you have to drive that through your description of events in the game fiction, and then deal with the paperwork as almost an afterthough. You have to address yourself to the characters, not the players:
Woah, Want, your followers' ranks are swelling every day since you did that miracle…
You're standing on the balcony watching the crowd of new arrivals camped in the courtyard of your compound when Dremer steps up beside you. His eyes are hard. "Boss, some of us have been talking. The stores are low, we don't have the food to feed all these new mouths. We want to know what you're planning to do about that." He's being pretty direct but still stopping short of giving you an ultimatum, but you can hear it behind his words, or maybe in the followers who sent him as their spokesperson. What do you do?
(That's a reveal future badness move, by the way.)
And then later:
So you're in the Contemplation Room with your inner circle when there's a disturbance in the hall. Suddenly there's this girl there, clutching at your robe and weeping. You don't recognise her, she's emaciated and obviously been starving. Then Crank is grabbing her hard by the leg and dragging her away and she's too weak to resist and she's shouting "Please! My baby is starving, we're all starving, we need you!" and then you realise she's just one of a crowd milling outside. What do you do?
(That could be a put someone in a spot move since its the Hocus's followers divided and in conflict, or it could be the threat move someone approaches, seeking help from the Affliction (Condition): hunger.)
Those two examples both build in the same direction, but that's not required either—the more different effects you show the more nuanced the situation becomes and the more interesting and meaningful the PCs' choices become. If the hunger issue above is happening at the same time that Crank is coming to Want all super-happy that now they have enough numbers to stage a retributive raid on a rival community, then Want is getting pulled in different directions and has the chance to shine with clever handling of the followers, or get pulled down into chaos by making the mess worse. That's awesome game fodder, that's Apocalypse World.
Casualties need to be made human too:
Dremer comes to you, face in hands. "Crank is dead." He's crying, you never seen Dremer crack an expression at all, and now he's crying in your arms. What do you do?
Half the gang is dead (mark them down a size by the way). There's blood and pieces of bodies everywhere. You're looking down at a lump of flesh beside your foot when the grotesque inhumanity of it suddenly snaps into a different perspective and resolves into half of Lala's face. She's staring up at you with one eye, like she's still asking something from you even in death. What do you do?
The battle with the raiders is over, they're all dead, but Gnarly, Last, Clarion, and Shazza are among the casualties too. You suddenly realise that that leaves only Kettle and Rice from the faction that was backing your push to claim the old power station, leaving the Isolationists lead by Corbett mostly alive. And then you notice how a lot of these bullet holes are in the back. What do you do?
The key is always to show, not tell, the change in the gang. Use an existing character or introduce a new character to not just indicate the change, but also to move the action forward by showing how this change pushes on the existing situation(s). In a small gang everyone should already be named, so when people die or leave, saying their names means something, maybe something big, maybe something small. For larger gangs there should always be names in the nameless crowd leaving, dying, or joining, so that you can start building on those names and interactions or lean on existing history with those characters to make the change meaningful and hint or show its impact on other developing threads and events.