Part of the problem surely would be the open advertisement of free drinks probably drawing way more people than expected, a ticket system would be beneficial, because despite how it might seem like a good idea to advertise the talents/company I'd definitely show up just to try a few of the drinks and then continue to only watch Tenma.
I wonder how many mixologists they have attending the bar to make the drinks. A huge backup line could just as well be an understaffed makeshift bar with amateur mixers on hand to make the drinks. Hard to account for capacity expectations and all, I'd like to know how they had this little speakeasy set up though because I'm almost sure I could've recommended a better workspace setup.