I have several users on my MacBook Pro. Will they all be backed up when using time capsule to backup to an external hard drive? Or will it only backup the user that it is logged into?

Yes, they will all be backed up, unless you explicitly choose to exclude them. in System Preferences > Time Machine > Options.

An older yet valid resource for most Time Machine questions that have persisted:

• Apple OS X and Time Machine Tips | by J. Pond (2013 archive)

http://oldtoad.net/pondini.org/OSX/Home.html

Good luck & happy computing!🌻🌤