I saw in the experimental changelog a few weeks back that they’d added in stuff for NPC mutations. And then stuff to give things to NPCs.
But I can’t seem to get my followers to inject themselves with insect serum so I can become their Hive Queen.
What am I doing wrong?
I dont think they can yet 
You could show us where that was sad, but as far as I know they won’t mutate yet. Or spawn with mutations.
They need to trust you. Currently the only way of earning trust is missions.
Once they trust you enough to eat stuff you give them (try having them use something cheap and safe, like a single charge of aspirin), they should also take mutagens. But in order to use serums, they need a syringe in their inventory.