Yerba Mate is an herbal tea of sorts. It is always drunk from a groud with a metal staw (bombilla) that has a filter on the bottom.It is not brewed like tea though. The gourd is packed more than 3/4" full with the Yerba Mate and a little cold water added. A thermos of hot water is then used to fill the little space left in the gourd with hot water.
The hot water is the sucked out and lots of noise should be made when the last drop of hot water is removed. (normally you tells your kids NOT to make that sound when drinking with a straw).
More hot water is then poured in and the gourd handed to the next person. When the flavor gets weak, the Yerba Mate is adjusted to make a new hole for the hot water in a fresh spot.
Many people are seeing walking around with a thermos and gourd. Many buses have a sign at the is forbidden to drink them on the bus (they don't want hot water spilled).
If you surf the internet you'll find that the Yerba Mate is mentioned as the national drink and very popular. That is true by the number of people you see with the groud and thermos.
However, look for it is a restaurant and you will find it absolutely impossible! The health department considers the sharing of them to be unsanitary and won't allow it in restaurants.