Visa letter is a bit risky unless you know the person really well.
You have to authorise to migraciones that you are responsible for this person during his/her visit and if he runs out of funds, you will be in charge of all his expenses and repatriation.
In the letter you will state,
I , so and so, am inviting this person, holder of passport number so and so, date of birth so and so, to visit me in argentina. he/she would stay with me. i would be responsible for her/him during his stay. He/she would stay her for 50/80/90 days .
he would visit the following cities etc etc
and he would have a return ticket .
bla bla
this would be endorsed by migraciones and the ministry of external affairs in buenos aires and sent to embassy of argentina in the home country of applicant plus a copy to the applicant.