Thanks @damnation but the issue is not that I can't extract the references from a document, but to extract them as linked citations. neither of the services you mentioned is doing that.
juris-m was able to extract the references from a document and generate a group library with the references still linked, so one could continue working on the original document without re-inserting every reference once again.

thank you @adamsmith
maybe I was not describing clear enough what I intend to accomplish: I have a document that includes references added with zortero, I do not have access to the connected library anymore.
What I want to do now is generate a group library for collaborative purposes with the references extracted from the document. These references should be linked between the group library and the source document so it would be possible to furter work on the document as if nothing happened. Exactly that was what juris-m could do (how I understood it) and reference extractor can't (at least that is what the website states), or I just do not get it.

no worries, thanks for pointing out the "select in zotero" option, though. did not notice that berfore, does not help with my current issue but will come in handy later for sure!