Do you need a visa when given an invitation to the usa??

Depends of you home country. You see some countries are automatically granted a 3 month visitor visa upon entry into the USA - you need to check on line if this applies to you.

For others the US will require you to apply for a visitor visa before trying to enter the US or you will be turned away at the port of entry.

If you wish to work in the USA you will need a work visa.

Just remember one thing...never never over stay your visa or there will be a 10 yr ban for you to return to the US