Phrase Usage Meaning – Good Idea vs Good Point

Both versions (with or without there) are perfectly normal things to ask. But if it is included, only the precise context (i.e. - what was said previously, who you're talking to, etc.) can tell us whether there refers to the location of the party, or the event itself.

Suppose, for example, you're talking to some friends in a pub, and one of them says "This is a really crazy place! Last week, two girls stripped off and danced naked on the tables!" It would be perfectly valid for you to say "Yeah! I was there! That was quite a night!"

The reason you can validly use there in that context is because essentially it can be applied to any place/time/event that's not here and now. Even though at the time of speaking, you're actually in the same location where the strippers danced, you're in a very different situation (they're not doing it now).

Contriving the situation even more, suppose instead, those two girls walk in and repeat the feat. You could say "That was crazy! I was here when they did that last week, too!".

In both cases you could use the other preposition. The "rules" aren't that fixed, and it very much depends on what you're emphasising out of different/same place/time/event. Not that I've ever been in either of those hypothetical contexts, but if I was I'd probably use there and here the way I've written it.

My reasoning would be that in the first case, both the time and the event are very different to "here and now". But in the second case the only thing that's different is the time, plus you've less reason to wish to emphasise the "not here and now" aspect of what you're talking about.

As regards "Are you there in the city?", you certainly wouldn't use there if you yourself were in the city at the time. Nor would you normally use it unless "the city" had already come up in the conversation. That's because in the city obviously refers to a spatial location that's not "here" (effectively, in that place, as opposed to in this place). It doesn't make sense to refer to that place if we don't know what place you're talking about, and it's confusing to have to wait for the words in the city to find out, so we don't generally phrase it like that. But if you had already mentioned the city, using there would be perfectly normal.

The two are very similar but I can give you a feel for the slight difference in connotation.

"At this point" implies that there is a continuous process going on. We stopped at 87.34% but we could have stopped at 87.35% and circumstances might be a little different.

"At this stage" implies that were are a few discrete phases where we could have stopped. We are in stage 3 of 5. If we wait a few more seconds, we are still in stage 3 and nothing has changed. It also might imply that something noteworthy changed in our circumstances between stage 2 and stage 3.

"In this point" is quite a different phrase, and is not common and generally used like the other two are. The example I can think of is if I was referring to points as objects that contain things. For example, if I was going through a presentation slide and in the third bullet point I say something interesting.