Hello yendruh,
As you allready said you can pull a Chatters list at https://tmi.twitch.tv/group/user/[CHANNELNAME]/chatters
Pull a list of Chatters and for every name you can run a check at https://api.twitch.tv/kraken/channels/[USER]/follows/[CHANNELNAME]
If the user is following the Channel you can visit https://api.twitch.tv/kraken/channels/[USER] and get the “_id” value, done!