It seems to be with the Salafi groups: Islam is Islaam, Qur'an is Qur'aan, Abu is Aboo, Salah is Salaah etc. You get the picture. Is it just to be more precise in English. You know if you are reading a Salafi article, book, website because of this style.
It has nothing to do with 'groups' or sects. It has to do with the arabic pronounciation.
Salah is a perfect example. In Arabic Salah is written like this: صَلَاة. The 'Sa' has a short vowel, and the 'Lah' has the long vowel. So, if it is written in English like 'Salah', it will make it seem that since the same 'a' is used both times, it has the same sound. But, it doesn't. The first one is short and the second is stretched out.
So, writing it like 'Salaah' makes the distinction between the sound meant to be said in both syllables. It gives the 'a' for the short vowel and the 'aa' for the long one, as opposed to a word like 'Salaf' which is supposed to have the short vowel both times. So, it is just to give a better sense of pronounciation (makes readers better understand how to say it properly).