Hey, first time DMing and using D&D Beyond. I previously had an account here and we made our characters for our one-shot on my account. How do I move those characters to the players accounts now that they have made accounts, if at all possible? 

You can assign the PCs to a campaign. You can invite the other players to the same campaign. You can Unassign the characters, and then the other players in the campaign can choose which characters to take over.

Nice, thank you so much.