Steps to replicate:
Log in as root
adduser testuser
adduser testuser -G wheel

Expected result: user is added to group wheel
Actual result: adduser: user 'testuser' is in use

Attempted fixes: kill any process owned by testuser; listing all running processes ps as root revealed none

Edit: further details that may/may not be relevant:

• Build compiled on OpenJDK 8 on Arch Linux, 1.16.5 branch, latest commit as of typing this
• Bug happened in survival; not tested in creative
• Computer contained 4x8MB RAM, 1x8MB HDD with Linux, 1x4MB HDD mounted at /home, Linux flash mem, 1x Redstone card, 1 bus connected to a floppy reader with a disk inserted
• Playing a modpack, Valhesia III with OpenComputers & Sedna .jars added to mods folder

I'm pretty sure this is either a configuration issue or limitation of busybox/buildroot. I'll try to look into this at some point, but if someone else wants to do some digging, that'd be much appreciated.

Found the busybox config. Will work via addgroup <username> <groupname> as of 7797c06.

Shouldn't this be in fnuecke/sedna?

Good point, though I've yet to test the bug in Sedna proper outside of MC. I'll do that sometime tomorrow and move the issue over if needed

I have the same issue on actual linux I think its intended to just be the second command? (adduser testuser -G wheel)

You can add a group later with: usermod -a -G examplegroup exampleusername
You can set the primary group with: usermod -g groupname username

Source: https://www.howtogeek.com/50787/add-a-user-to-a-group-or-second-group-on-linux/