I’ve seen sites listing this as the way to add a FreeBSD user to a group:

# echo "DON'T DO THIS"
# pw usermod [username] -G [groupname]

You almost certainly don’t want to do this. This command removes you from every group except the one you’ve listed. This has grave implications if you were a member of wheel before, because now you can’t elevate to root.

Instead, you want this:

# pw groupmod [groupname] -m [username]