How can I find out which known stars lie within or close to M8
Have a look at the following references - found relatively easily using the following query in the NASA ADS site.

UBVRI and Hα Photometry of the Young Open Cluster NGC 6530
This paper by Sung &amp; Bessell (2000) contains an extensive photometric catalogue. The main table can be queried interactively.

The Lagoon Nebula and its Vicinity
This is a review article by Tothill et al. (2008), from which you can get pointers to most of the significant literature (pre-2008) on the region.