This discussion largely focuses on "yet" versus "already." It is a context thing and, being a stand alone sentence, there is no context. To my Chicago ear "Are the others already there?" implies the others are earlier than expected whereas "Are the others there yet?" implies the others are running late.
It is about stress in the sentence. "er" does not stress the specific relative location, it simply means here/there as per context - as explained by vam1980. If you want to stress the specific location you can replace "er" with "hier" (here) or "daar" (there) which gives it a crisper meaning.
The difference between ‘already’ and ‘yet’ is pretty subtle. See http://www.pearsonlongman.com/ae/azar/grammar_ex/message_board/archive/articles/00334.html for some discussion. I'm not sure how closely this matches the difference between ‘al’ and ‘nog’ in Dutch.
Why Lonja? Others have ALREADY explained why yet is incorrect, YET no one has explained why yet is far better.
To me in English already implies speed, being early or on time and completed events. Yet implies slowness, delay, incomplete events. I presume it is the same in Dutch; but do you feel differently?