Riverside will save an mp3 file of your recording and a separate mp3 file of your guest(s) recording(s) to your computer.
If you wish to have guests on your podcasts who will not be in the same room as you, you’ll need a remote recording service. I recommend Riverside. There is a free and paid option and requires that both you and your guest use Google Chrome and have a good internet connection. The free version allows you to 2 hours of separate audio & video per month and have one guest (meaning you + one other person) per recording. Riverside will save an mp3 file of your recording and a separate mp3 file of your guest(s) recording(s) to your computer. You can then take those files and drag them to GarageBand for editing.