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.
In most situations they are interchangeable. According to this NGram, so far seems to be significantly more common than up to now.
In situations where something is coming to an end, there may be a preference for up to now, whereas so far implies that the situation will continue:
Meals in the staff canteen have been subsidised up to now, but from January the subsidy will be withdrawn.
The festival has been pretty good so far- let's hope it stays that way!
Best Answer
Yes, there is a difference. The expression "[things] have been [done]" implicitly means "to this date". Adding "so far" might seem redundant, but it's not! It adds an additional implication that what has taken place up to that point may not continue to take place, or if it does continue, may be adjusted, embellished or taken to a different level at some point in the future.
In your example, it seems likely (from the limited context) that "so far" is being used to flag a change of direction. The addition of the phrase gives the reader advance notice that you are about to leave the familiar evaluation framework behind and embark on a new stage or level of the process.