There are several ways to share your Mii character with other people.
Transferring Mii characters to other Nintendo 3DS systems, using a local wireless connection
This option is ideal when both Nintendo 3DS systems are in the same location. Open Mii Maker through the HOME Menu of your Nintendo 3DS system, and select Send/Receive. Choose the option Mii Maker. Do the same on the other Nintendo 3DS system. The first person chooses Wait for Players, while the other chooses Search for Players and selects the first person's Nintendo 3DS. Both systems can now select the Mii characters they wish to send to each other.
Transferring Mii characters to other Nintendo 3DS systems using QR Code patterns
QR Code patterns contain all the data needed to recreate a Mii character on another Nintendo 3DS. QR Code patterns are saved as an image on the SD Card of your Nintendo 3DS system. You can transfer these images to your PC using a card reader and distribute them among friends. Your friends can then scan the QR Code to receive your Mii character on their Nintendo 3DS. You can create or scan a QR Code in your Mii Maker, by selecting QR Code/Image Options. Either choose Save Mii as QR Code or Scan QR Code, depending on what you want to do.
Saving Mii characters as images
If you want to create an image of your Mii character to use elsewhere, you can do this in Mii Maker, by selecting QR Code/Image Options and then Save Mii as Image. Select if you want a picture of only the face or the entire Mii character. The image is saved on the SD Card of your Nintendo 3DS.
Sending and receiving Mii characters via StreetPass
You can also send and receive a Mii character using the StreetPass function of your Nintendo 3DS system. You need to navigate to StreetPass Mii Plaza in the HOME Menu and choose the Mii character you wish to be transmitted to other systems. Then, when you pass other Nintendo 3DS users with your system powered on or in Sleep Mode you will automatically receive a Mii character from the other user and they will receive yours. If you wish to save the Mii character in Mii Maker, simply select a Mii character from the line-up in StreetPass Mii Plaza, press the A Button and then choose the "Mii Maker" option.
Please note: You cannot edit Mii characters that have been received from someone else and do not have copying allowed.