How much risk your horses are at depends on a lot of things: type of grass, actual size of the pasture vs. number of horses, weight and condition of the horses, genetic predisposition, etc. Welsh ponies and Arabians are fairly high risk genetically, and so are other "thrifty" breeds, like other types of ponies, mustangs, etc, and horses who are overweight are more prone to laminitis and founder as well. Sudden introduction to lush grass is also a problem. I use a dry lot and limited turnout, but my mare already has chronic laminitis. She is a 1/2 Welsh, 1/4 Arabian cross, and was kept on a 40 acre fescue pasture for several years by my parents while I was in college, became obese, and eventually foundered. She is now very sensitive and requires a lot of careful management to be sound.
I have found this to be an excellent reference: Katy Watts | Safergrass.org
This is also good, but focuses more on treatment and management than prevention: Laminitis Help
Supplementation with magnesium and chromium can also be helpful for at-risk horses, particularly if they have a cresty neck. I'm really happy that you are finding this out before you have a problem--laminitis is not fun for anyone involved.