But there is a work around. While you can't send GarageBand projects from one iPhone to another (at least I haven't figured it out yet), you can send tracks to one another to include in each other's projects.
First Step: Send the track to the other person's phone via messenger like so...
Second Step: Receive or send track to other iPhone (simple right?)...
Third Step: Click on the track to open it in its own screen so it looks like the below and click on the upload button on the bottom left...
Fourth Step: Click on Add to iCloud Drive
Fifth Step: Pat yourself on the back, you are halfway through this exercise.
Sixth Step: Choose "iCloud Drive" as the location to add item to.
Seventh Step: Open new or existing track you want to add to.
Eight Step: Make sure tempo or BPM matches track. This track was 110 so...
Ninth Step: Hit the Loops (little lasso at the top right) button to find track.
Tenth Step: Click on Audio File and click Import from iCloud Drive.
Eleventh Step: Click on the Track you want.
Done and Done...you now have a track in your project.
Any questions?...class dismissed.