But wouldn´t the present perfect imply the present tense: people I know?
Not necessarily. If I remember correctly, grammar books usually get around this by saying the present perfect implies "some effect" on the present. This is also why the present perfect is difficult to teach. For example, if I say "I've been with 200 women", it doesn't mean I am with a woman at this moment.
In other words, when I say: "I have known my friend since we went to school." does that not mean: "I still know him." so that it would properly do without the latter?
In this case yes, but "since" and "for" sentences are more straightforward.
Or did you mean by using the present simple in addition to the present perfect to render some extra information or to put emphasis on some aspect that is not implied in the latter?
Think of it this way: If I say "I've been to Hungary 3 times" it's not clear where I am now. Same with "I've known many English speakers". With only that sentence, I might not know any at the moment.
So I meant to communicate that not only have I known many native English speakers over the course of my life, but I also know them currently. (and only two have said "I haven't a lighter" :D)