Not always. If you personally know the person who created the flashcards in question, and if they invite you to their Class, then you should be able to study those flashcards completely for free.
If you have joined the Class of someone you know, but the Decks appear "locked" for any reason, simply contact the Class's creator and ask them to give you Study access, which will remove the lock.
Otherwise -- if you'd like to study flashcards that other users have created -- then you can only "demo" the first Deck in their Class, and you will need to upgrade to Brainscape Pro in order to study the rest. This helps Brainscape pay the bills and also provides important compensation to the flashcards' creators who did all the hard work!