Well she said that her friends in her 20s agreed with her. The OP is out of her 20s and seems to think a few dates is a good enough reason for exclusivity. I suppose there’s just a lot of different opinions on these types of things.
That’s the thing, I’m not used to that in Spain. In the US it’s how the dance is (traditionally) danced, here it’s more of a group thing. The details vary by region: in most places, it’s often a matter of couples meeting when one of them joins the other one’s group of friends, or of getting both groups of friends to go to the same places, perhaps eventually merge. In Euskadi, usually it’s boys-groups and girls-groups, and never the twain shall merge: meet, yes, merge not even after half a group of boys has married and had children with half of a group of girls.
My brother and his wife were considered “a couple” for close to 7 years before they got married. He’d gone to her house for the first time before she’d even accepted that well, ok, her friends were right and he was indeed her boyfriend (he and his BFF had gone to pick her up and Bro’s future parents-in-law had made them go in); they’d been alone there for the first time a month after she had (and almost given her mother a heart attack when the parental units returned “early” and found them in the kitchen, where Bro was cooking - and as Bro says “I really wonder what kind of dumb did they think we were, if we’d wanted to use a bed we wouldn’t’a needed to do it in their house!”); they went out on their own… uh… never? I do know some couples who “went on a date”, with a show/movie and dinner at a posh place: once they were married, as an anniversary celebration.
I went to quite a few dates in the US, there guys didn’t propose group activities until after we’d gone on a few.
While the “group thing” is certainly normal on the U.S., it’s generally the field of younger people. Once people start working and getting married, few people have much time for the group thing often enough to makeit the exclusive way of meeting prospective partners.
Also, by the end of your twenties, you tend to have worked your way through the group. 
“worked”, is that what they call it? 'Cuz that would be helpful next time my boss asks if I’ve been working.
Since starting from a group of friends seems to be the norm, how does one transition from “we’re part of a group of friends” to “we’re interested in sex/romance” in a way that’s both smooth and reduces the chances of misunderstandings?
Few if any middle-aged people do the “dating in a group” thing. I do have a group of friends, and there are a few single men in the group, but let’s just say they’re single for a reason. If an appealing eligible man joined the group, there would be a lot of competition for his affection–so much that the group might no longer be a group.
Most of my friends have resigned themselves to online dating or to staying home wondering where the single men are. Don’t get me wrong–we have full lives and aren’t moaning that we don’t have a man. But once in a while we want one, and they don’t seem to be around. The single men I know are happy to be single and prefer dating around but not settling down. This has been going on or years; it seems few of them are interested in a “relationship,” while most of the women I know are. And many of the single men in their 50s and even 60s I know want to date women who are 20 and 30 years younger than them–but that’s a different story. I often get attention from men in their 70s who are only a few years older than my father, or from men in their 20s and 30s, who seem too young to me. But that’s another story, too.
Great line by an American columnist in (I think) the Guardian that sums up how British people do it: drunkenly meet a party or bar, drunkenly have sex, then either move in together or never speak to each other again.
Pretty accurate in my experience.
pdts
So, the British equivalent of “dating” is “getting drunk.”
My “culture”, such as it is, would be described as highly educated upper middle class urban professional types.
In college, people typically didn’t “date”. For the most part we would just gather at bars or fraternity parties and “hook up” with varying degrees of success. If you liked the person, you might hook up again. The idea of meeting someone, going on a bunch of formal “dates” and then deciding if you wanted to have sex with them seemed a little strange. Although that is not to say that every first hookup resulted in sex.
Kids in the 80s and 90s when I was in high school / college did “go steady” in concept, although the term was pretyy dated. Usually we just called it “going out” or the other person was your “boyfriend/girlfriend”. This condition presumed monogamy.
Some conceptual levels of relationships:
A “hook up” - basically a one night stand with someone you may or may not ever see again.
“Booty call” - Repeated hook up with the same person with no prerequisite condition for emotional attachment or monogamy
“Friend with benefits” - Basically a relationship with the commitment of a booty call but more emotional attachment.
“Dating” - Presumes some level of serious relationship with at least some intent to explore a deeper commitment or end the relationship. In comparison, the previous levels of relationships can sort of string along with no commitment indefinitely.
“On again / off again” - Indicates a serious level of commitment intent, but an inability to fully commit to either marriage or ending the relationship. Typically the relationship will be dysfunctional on some level.
“Boyfriend/girlfriend” - Basically a traditional monogamous relationship where at least the intention is to naturally progress from a series of dates to living together, marriage (“fiance”), etc on a typical timeline.