I will add some supported information to the answer and comments, perhaps more for future reference. There are guidelines for author ship. The ICMJE (International Committee of Medical Journal Editors has written up guidelines for authorship and contributorship based on the Vancouver Protocol. You can also see the post Paper contributions and first authorshipPaper contributions and first authorship The requirements for becoming an author are quite strict. Based of these guidelines you can see whether co-authorship should be reasonable.
You can also check out COPE's How to handle authorship disputes:a guide for new researchers and the American Psychology Association APA Student’s guide for thoughts on authorships and disputes particularly from the perspective of a young scientist.
To cap off, your situation is far from ideal. To evaluate possible co-authorships must be made early on. You will inevitably experience people who will bully their way into a paper (pressured authorship) and in some cases such behaviour is more a tradition than abuse (but nevertheless wrong and unethical). But remember that missing to add an author (ghost authorship) is also unethical. What you can do in this case is to contact the editors of the journal to seek their advice. Suggestions to retraction of articles is not something they take lightly. You do, however, need to assess the authorship issue carefully along the guidelines given in the examples above. Whether someone is missing from the acknowledgement or not is cause for any drastic measures. The acknowledgement is the only part where the authors can add thanks etc. as they see fit. It is not even necessary to have an acknowledgement, although that might appear odd to readers.