If you are there, since you are able to see (unless you are using a camera), it may be expected to use "here" - instead of "there"

der -> her

I notice a little bit odd sounding in my language (after translating, knowing Norwegian has "feeling" for that issue), so I am proposing this change. However, if it sounds OK in German, just please ignore this suggestion. Thanks. Both variants are obviously possible. Just a nuance.

How can you know that I am there? Maybe being here, I can see what is happening there.
I think that's a quite normal situation.

As far as the German sentence is concerned, I admit that it is ambiguous.

"Alle waren schon da." can be interpreted in two ways:
1. Alle waren schon dort. (= Everybody was already there.)
2. Alle waren schon anwesend. (= Everybody was already present.)

So are you going to add a second Norwegian translation?
Maybe: "Alle var allerede til stede." or someting like that?

I see. I was wrong. Being there, doesn't prevent you from saying, later on, that they were *there* waiting for you. I was imagining the situation when this your sentence is uttered in that moment when you were standing before them who were waiting for you. And then you would say that "they are waiting *here*". So, it is correct sentence in any way, if it is considered to be said after the event, what use of past tense clearly indicates. I was confused a little bit. I will check again my translation. Thank you. (this was about your first response).

About alternative sentence, there is no need. We have several examples with "til stede" on Tatoeba, I believe.

If my first response is that one I deleted meanwhile, I beg your pardon. I was just confused in the first moment and entered my comment to fast, not having read even my own sentence first. I guess you've got it via e-mail nevertheless. So please ignore it.

I don't look too much at Tatoeba emails (they are being automatically put into an archive).

I meant on your first, still standing, response - which opened my eyes for the fact that we are actually speaking in the past in the sentence (I guess your use of continual present tense form made me realize it - having being contrasted to the past tense of the original sentence). Somehow, discussion turns always to be fruitful, even when started around wrong things. If we are open to listen, we might learn something new.