Usermod.

Hi @WarrenUK ,

From man usermod(8):

NAME
usermod - modify a user account

SYNOPSIS
usermod [options] LOGIN

[...]

OPTIONS

-G, --groups GROUP1[,GROUP2,...[,GROUPN]]]
A list of supplementary groups which the user is also a member of. Each group is separated from the next by a comma, with no intervening whitespace.
The groups are subject to the same restrictions as the group given with the -g option.

[...]

So, the command basically adds the user to groups in the system.

I hope that helps,
Luis.