Bot can auto approve as well as auto disapprove new channel join request.
/start: To start the bot.
Just follow below simple steps.
Click on START to start the bot.
Add bot as administrator with 'invite users' permission and forward any message to bot
Now all new join requests will be auto approved.
Please note that the bot will send notification to the user saying the request is auto approved. If you don't want the user to know about this use this bot @AcceptRequestBot.