How can I surreptitiously poison someone?

[Magdela]

Your teammate in the back is a waiter, he makes your drink custom.

Have the teamate give you the drink, which had the potion in it. After that, you hand the target the drink. When he drinks it and turns invisible make your best attempts to follow him around and separate him(from the crowd...the alternative is messy). Knock him out and hide him in plain sight. When he is knocked out use a needle tipped with drow CONTACT POISON!!!

Edit: (not my idea) Give them a cake and a drink covered drow poison. Bluff them that the antidote is in the cake and that the drink was poisoned. In reality the cake is poisonous and the drink was actually filled with alchemists fire.