Depends on what you mean by "rock climbing community".
- University mountaineering communities (Bogazici Uni Mountaineering Society, İstanbul Technical Uni. Mountaineering Soc., Yıldız Technical Uni. Mountaineering Soc. etc)
- Private mountaineering communities (http://www.dag.org.tr, http://www.zirvedagcilik.net etc.)
- People who climb in the gyms (BoulderHane & Boulderistanbul)
if you are looking for training the gyms are the best option, if you are looking for some fun time and learning stuff university societies are the best option (some of them gives lectures in english), if you are not a student and just looking for people to go rock climbing with private societies would fit you the best.