I would contact the authors first, pointing them to your paper and simply asking if they are familiar with it, and indicating – with no judging/accusing comments! – that their approach is similar to your published work. Then two things might happen:
They will contact the editor and he will handle it; either they will have bad luck and his decision will be to reject the paper, or simply there will be a "note added in proof". In the latter case, you'll gain a citation, too.
They will neglect your e-mail, either not answering at all, or giving some vague excuses that you might feel don't lead anywhere. In such cases, write directly to the editor of the journal, pointing him to your article and emphasizing the similarities between the two and that yours was published long before their submission (and maybe noting also that your two groups haven't collaborated).
An option might be to also write a short comment (I guess 1 page will be enough) to be posted on arXiv, just so that the community knows. But first contact the authors, then the editor (if necessary) – in the end it's his decision what to do with the submitted paper. I think there's only a slight chance that in the end you won't be satisfied with how the thing was handled – at least you will know to think twice before submitting to this journal again, and the community will be warned by your comment on arXiv. But don't jump too far ahead, first see how the situation will be dealt with.