Forum feature and external code

MIT App Inventor Help
2

Did you search the FAQ ?

Not sure what you mean by forum feature, but if you mean something like this in an app, then no (but you could use a webviewer to access an online forum, or try out the many different chat options:

For Google Sheets, also check the FAQ:

or read below:

AI2 and Google Drive/Docs/Sheets
 
HOWTO: Query Any Google Sheet with a Web App.
UPDATED! HOWTO - Upload Any File to Google Drive !
HOWTO: Use AI2 to directly submit Google Form Data
HOWTO: Google Drive - Multiple Files Download (file by file)
HOWTO: Download Google sheet as CSV
HOWTO: Send Data to Google Sheet with Web App
HOWTO: Show Image from Google Drive
HOWTO: Upload Image to Google Drive using base64 and Google Apps Script Web App
HOWTO: Upload Image to Google Drive using base64 using Camera Image
HOWTO: Get Data from Private Google Sheet with A Google WebApp
HOWTO: Get the ID of Image after uploading using base64
HOWTO: Upload CSV or List to Google Sheet
HOWTO: AI2 - Google Sheets - Web App - Create/Read/Update/Delete
HOWTO: OCR Images and Camera Pictures with Google Docs
HOWTO: OCR pdf and image files using Google Docs OCR function
HOWTO: Upload "Any" File to Google Drive
HOWTO: Use google web app and google query to download filtered output as pdf from Google Sheet
HOWTO: Use Ai2 to Delete Images (or other files) on Google Drive
HOWTO: Take Photo, get GPS Location and Address, upload Image to Google Drive, and save Location data to spreadsheet 
HOWTO: Select a Single Row from a Spreadsheet, and howto EDIT that data