He was present along with Solozzo, Sonny, Tom, Clemenza, Tessio and of course Vito. Why was here there though if he wasn't considered high-up in the family?

Good Question. I assume since he had strong family ties he was allowed to be there. I assume Fredo is at least a made man. although they dont reference being made in The godfather

There are many differences between the novel and the film. In the novel, only Vito, Sonny, and Tom meet with Sollozzo. Thus, FFC exercised the latitude to include several more people in the meeting. To that point in the film, there is no indication that Fredo is not "high up" in the family.

I believe the reason Vito had everyone present at the Sol meeting was to hear him say "no," firmly and finally. He knew that drug profits engender greed. He knew Sonny and Tom were hot for the deal. So, he wanted everyone, including Fredo and the caporegimes, to hear him say, "My 'no' is final." That way, there'd be no ambiguity, no "ch***ks in his armor," that would allow greed to take over. Also, he was, in effect, firing a shot across everyone's bow: If you decide to dabble in drugs despite me, everyone else who witnessed my "no" can turn you in.

But also for dramatic effect. Everyone in the room was a major player in the film whose presence would continue throughout the film. Thus, the audience is given a second introduction to them, second to the wedding.

That's true. It is probably much more important for Clemenza and Tessio as they were seen in the wedding scene, but their importance was not firmly established until the Solozzo meeting.

At least Fredo had the good sense to kep his mouth shut.