How to add people who belong in groups to a region: /region addmember g:GroupName. Since World guard can already read groups, could it also read clans? Currently this doesnt work: /region addmember c:clan. This is the only shortcoming and the only reason i'm currently not installing it.

This must be make from the Worldguard Team. The SimpleClans Plugin has all for implement this. In my Plugin i make this for the Protection.

SimpleClans.getInstance().getClanManager().getPlayerByClan ore something other you want.

/region is not a SimpleClans command. Like reini said, If you want /region to recognize clans you need to ask WorldGuard to add SimpleClans support to their command/plugin.