can someone invite me go lowyat guild. My ingame is lchee0909 with L, not i.

waiting for invite to lowyat dota 2 guild, ingame nick Lchee0909

I watch dota games and i see teams putting sentry ward at certain high points to block creep spawn. I know observer ward can block, but how can sentry ward block creep spawn? Cause sentry ward does not give vision if u got nothing near that area. Anyone can let me know why they put sentry ward near creep spawn area and somehow it still able to block creep spawn?

thanks for the guide. So question is, is sentry ward alone considered as unit to block creep camp from spawning? I know observer is able to.