Let's try to be clear...
1-Renfe Joven card +50 is a card for young people (under 30 years old) that applies a 50% of discount on the full fare on all long distance trains, such the Alvia train. Forget about cards and discounts, the real discount at www.renfe.com is the Promotional fares (Promo fares ) and the Mesa fares, that applies up to 70% of discount and leaves the cost at almost nothing. The key point to obtain those fares is to book super early, and to book fast, as those fares are limited in time and number.
2-The theory says you can book from 62 days before the train departs. As I can see at www.renfe.com, you can book now that route up to the 28/05/2018 , so that means there is still a month to pass. What spaniards do is to check the website every day. If it is available, you buy it, if not, well, try tomorrow.
As you are 5 persons, I would try to do 2 purchases: 4 persons (you could get a superb Mesa-Table fare) and 1 person. Everyone gets a seat, but 1 person might have to travel separately from others. You can change during the journey the seats so no one is lonely all the time. Don't worry, the trains in Spain are super modern and you can walk all along the train.
Mesa fares are super great for families, it is super simple: 4 persons get seated all around a table: 2 seats are facing forwards, 2 seats are facing backwards. And the table is great as you can do activities: play cards, chess...whatever you like.
Unlike the US, when you buy your ticket automatically you have the coach number and the seats, so everyone knows where you will seat exactly...no wait for the train conductor, he/she only checks that you have seated at the right seat and you have a valid ticket.
3-Forget about promo codes, just book early in advance. If you follow my advice you will have a superb fare.
4-The main train station (and only) for trains that use the High Speed line (250 km/h for ALVIA trains...super fast!) is Barcelona Sants. Once you arrive at Barcelona Sants, you can take the Metro or Rodalies. Rodalies (catalan name for Cercanías) are the commuter trains of Barcelona, that stop at key points like Passeig de Gràcia, Plaça Catalunya (this is the top of Les Rambles). These trains are super fast, and they run very frequently all day (3 minutes)...and why am I telling you about this stuff? Because with all your train tickets from San Sebastián you have free of charge a free ticket on Rodalies once you arrive at Barcelona Sants, so you can go wherever you like into the Rodalies network. And that includes Plaça Catalunya as well. Once you get your tickets (print at home or show them on your mobile, super easy) you will see there is a code called Combinado Cercanías in every ticket. Once you arrive at Sants station, go to any orange machine, touch Combinado Cercanías, type the code and magic...a free ticket comes for anywhere you like inside Rodalies network. Do this for every code, once you have your ticket go to Platform 8 at Sants and take any train you like, in 3 minutes you will be at Plaça Catalunya!
The best website is www.renfe.com , the official provider and the only one that sells mesa fares as well. Other retailers do not sell that fare. Once you end your purchase, you will get a PDF in the mail provided. Print it, and you're all good to go.
While you wait your train to be opened for purchase, I suggest doing 3 things:
1-To set up a PayPal account: Renfe website doesn't like the security standards of US cards...so set up a PayPal account with your card, and use the PayPal option at www.renfe.com
2-To be calm when buying at renfe.com . It is a challenging website, but once you have the trick it is very easy. Use google translate if necessary, and practise a simulation of a purchase. Note that all purchases at renfe.com can be cancelled up to 2 hours free of charge...everyone makes mistakes sometimes, and more spaniards!
