Speaking from my experience as an AL DM, necromancer horde wizards tend to draw the ire of the rest of the table for excessive time consumption. AL is generally played in a time-constricted environment. If each of your turns take 5 minutes to handle all of your horde's actions, expect the DM and other players to begin disliking your character. It may also lead to unexpected difficulty spikes, as the DM has to ramp up encounters to factor in all of your horde. Also, there's no guarantee that one DM will accept the undead you have left over from your adventure with a different DM, nor do they have to accept the proposition of spending DTD to start the adventure with a horde.
Simply put, my advice for playing a necro in AL is don't. Your mileage will vary wildly from table to table, and no one will want to play with someone who drains time during combat. If you must, then play it in an AL Home game. Plus, it's a rather complicated and resource-heavy character for a newbie to play.
Lastly, you can't transfer ownership of a character between players. Your friend will have to start at 1st level, unless the Warlock was his to begin with at 1st level.