I'm looking for the best prices for getting around Barcelona. We will be traveling as four adults and one kid (16 y.o.) and we have accommodation in Barcelona city center for ten days. Also we would like to make short trips to nearby towns of Tarragona and Figueres (leave in the morning and return to Barcelona in the evening).
So my questions are:
What would be the best conditions for us to use the metro? I have found this Hola BCN! travel card which looks only marginally good as it offers unlimited metro and bus access for one person for 5 days for 32€ which translates to average 3 journeys per day if compared to one-time tickets (2.15€). I heard there are some special tickets that can be used by groups of people as long as they enter at the same station but couldn't find any info online. Is this true? Are there any other options?
What are the options for trains to Tarragona and Figueres beside buying single tickets at the station? Are there any discounts based on the passengers' age (most of us are under 25 years)?
The card which you have seen is the T-10 (http://www.tmb.cat/en/sistema-tarifari-integrat/-/ticket/T10), which costs €9.95 and is valid for 10 journeys on the metro, buses and trains, including up to two changes (eg train to metro to bus, or bus to metro, but not metro to metro). These are valid for more than one person to use simultaneously, as long as the group does not split up at any point. You can also get day travel passes for €8.40, valid for one person for as much travel as needed in one day, or, if you think you're going to do a lot of travel, you can get a T-70/30 (http://www.tmb.cat/en/sistema-tarifari-integrat/-/ticket/T70-30) which is valid for 70 journeys (with changes, as per the T-10) in 30 days, for €59.50 (the T-50/30 is only valid for one person to use at once while the T70/30 can be used by more than one person simultaneously). The T70/30 might be the best solution for you as it would let your group do 14 journeys for the cost of two five day passes. The T-70/30 is also valid to and from the airport on metro line L9Sud, which the T-10 doesn't cover.
If you have a Euro<26 card, you can get a 20% discount on regional train fares (this includes the Avant trains to Figueres). The card issued in Catalonia looks like https://www.carnetjove.cat/bundles/undfmain/images/img26.jpg?v1.3.8, but the important thing is the "EC" logo. If you don't have one then unfortunately there are no discounts for being under 26, and adult fares are payable from age 15 up. If you read Spanish, more information on this is at http://www.renfe.com/viajeros/tarifas/carnet_joven.html. Alternatively, you may be able to get cheaper fares on long-distance trains at the Renfe website (for Tarragona, make sure you don't book to Camp de Tarragona, which is the high speed station but is around 12km outside Tarragona itself).
In addition to Edd's great answer, I would point that:
Cards are sold by numbers of zones. Zone 1 includes Barcelona and neighbouring cities.
The most used card by local people is T-10, since it's not worth moving to a more expensive card if you aren't doing more than 10 trips a week. Anyway, beware that it doesn't include underground to airport and that usually tourist do more trips in less days than local people.
When using cards other than single travel ticket, journeys allow for commuting between networks: underground, bus and two rail networks (FGC and RENFE). With zone 1 cards 1:15 hours is allowed to commute up to 4 different networks or bus lines (time is measured since you entered first network until you enter last one). This can be used to go to two different places (or to get to one place and come back) using just one trip from your card. Anyway, this can save you just a few trips unless you are trying to visit a lot of places spending little time on each one.