Go to the osu! settings, then skin and press the "Export as .osk" button, your skin will be exported into the Exports folder (osu!/Exports) then upload the .osk file on websites like Mediafile, Mega.nz or send.firefox.com. Then get the shareable link and send it to your friends, they have to download the skin and import it into osu! by opening the file or by dragging the file into osu!