Source: https://superuser.com/questions/77617/how-can-i-create-a-non-login-user

Create new user

adduser --system --no-create-home --group --disabled-login $USER

• Create a user with a UID in the “system” range (not much different otherwise)
• No home directory
• Create a group for the user (same name)
• Prevent the user from logging in normally (can switch from root though)

Create Group

addgroup --gid 2000 $GROUP

• Can manually set the group ID with --gid

Add existing user to another group

usermod -aG $GROUP $USER

• -a ensures the group is appended to the user’s list of groups instead of replacing it.