Hi, there. I started running a game for some friends and I created their character sheets on here. One of my friends has now created his own account here and I was wondering if it was possible for me to transfer his family's characters to his account from mine? Thanks in advance!

You can.

Create a campaign, and upload those characters as Unassigned Characters. Then, invite your friends in the campaign with the campaign link, so they can claim the characters for themselves.

Thank you, filcat!