Translation:Friends are good.
In those examples it doesn't make a difference, they use one or the other and it doesn't matter. I/ga is your basic subject ending. Eun/neun is the subject ending or the topic ending, when you want to emphasize who the subject is in comparison to another one. For example you want to say "as for me, I like chocolate". Because somebody else said they liked something else or that they didn't like chocolate.
You are correct in stating 은/는 is used for emphasis. However, it has other uses as well--sometimes just to make a general statement, some times to differentiate, other times, just as a follow up, when the topic has already come up in conversation, and I expect there are more. In order to really help us learn to differentiate between 이/가 and 은/는, DL would have to give us more context. Throughout the course one or the other is presented as the "correct" answer, when either could fit, depending on the situation. But, to DL's credit, they are gradually correcting them as we contnue to help.
좋다 to be good, 좋아하다 to like. So 친구들을 좋아합니다 means "(I) like friends", but 친구들은 좋아합니다 can mean either "friends like" or "friends are liked", depending on the context, since the topic marker 는 only relates a statement to something, but doesn't specify the manner of that relation like the subject or object makrers do.
As for the 이/가 vs. 은/는 debate, I always take sentence length into account. I'm fairly new to this, but I make sure if the sentence is short and only has one topic, I use 이/가. Thereafter, you may remove the topic word if you stay on that topic (see how that works out?), or I just use 은/는 because saying (thing)이/가 again would sound too emphatic or redundant, I assume. Korean is all about nuance. Try not to take word order and direct translation as seriously as the feeling of the sentence.