You can tap Add a Mii on the Manage Mii Characters screen, or tap the Mii icon on a Mii Racing Suit, and then tap Add Mii on the bottom left of the Choose a Mii screen.
There are two ways to add Mii characters:
Add them via a Nintendo Account (you will need to log in on the Nintendo Account website):
1. On the Add a Mii screen, tap Choose / Create a Mii.
2. Select a Mii character you've already created, or create a new one.
Add Mii characters using Mii numbers:
1. On the Add a Mii screen, input a Mii number that someone has shared with you into the Mario Kart Tour Mii number box.