Hi!
I have a windows forms app that does that. I could personalize a little for your needs, but this should be discused.
Right now, you authenticate with your telegram user (this is mandatory by telegram*), then you type in a box the Username of the channel (can be other identifier) and it tells you the count of members of that channel. You CAN'T access the members because that needs user admin privileges.
I have this program working right now
* You have to get a telegram api and hash from their developers web in order to let the app access this data, this can't be overriden