Can I log on to an iPad already registered to another user?

Only if you don't mind blending your a data with theirs, or they do not mind you deleting their data from the device as part of the sign-in process when you sign in. Apple's mobile devices (iPhones and iPads) are not designed to have multiple users. If you want to do something on another person's device, use a web browser on the iPad and sign into your account there.

Only if you wish to blend your information together. To use someone else’s iPad as yours necessitates first logging out of the Apple ID and factory resetting the device.