I am trying to add sudo to default guest account

Some one said to follow this process, but I am noob with usermod, and I am very confuse on this one. I dont know where to start.

You could use sudo and the usermod command from an account that does have sudo access to add theadm group (for 12.04) to the account. Don't forget to use the -a option to add the group rather than replacing the group list.

I try this comands in terminal:

id tom

And it sais.

uid=1000(tom) gid=1000(tom) groups=1000(tom),4(adm),24(cdrom),27(sudo),30(dip) ,46(plugdev),106(lpadmin),117(sambashare)

I am trying id command but with guest account, but sais account do not exist

id guest (account do not exist) I dont know wich is the name that Ubuntu refers to that account

I am lost on this one. Can some one can go on high detail about what is the correct command I need to put
What command I need to put in the terminal in order to do this? I am getting the felling thats is not even a long command. And it must be so simple but as a noob I just cant see it.