You can either allow the importing of player characters under Game Settings and have the players use the Character Vault or Roll20Characters to transfer their characters, or, since you have a Pro account, you can use the Transmogrifier to copy their characters to the other game, invite the players to that game, then once they join assign their characters to them. That second way (using Transmogrifier) is probably faster and simpler, since you wouldn't be waiting on the players to complete the transfers. Edit: Added links.
