Going to be in Vegas mid February and looking for some guidance.  It's been a while since I've been there, seems that key cards are the norm now in order to access elevator towers.  Outside of the somewhat non-discrete, "meet me at the bar" connection, any advice or options I might consider for getting a key discretely to a companion?  Will likely have some co-workers around and the thought of meeting in the lobby bar just won't work.  

Find a more discrete meeting spot than the bar or lobby, set up a signal of some sort (tug your ear, rub your eye, wear a hat). Floor maps of all the hotels are available online, shouldn't be too hard to find a spot, maybe behind one of those giant planters that are ubiquitous in Vegas hotels.

I fully understand this problem...I have met them in the parking lot, or at the Uber/Lyft drop off.  Handed them the key and told them the room.  I have also met them at the elevator, a little more risky but usually brief and no problem.

Some things I have done...
Scope out a public seat. In a lobby. Near a lounge. By a coffee shop. Make eye contact with your date as she nears. Smile. Accidentally leave a keycard in the seat. And have her follow.

You could meet her in the gift shop and just give her the key in passing. It works every time.

Sorry, but there are really no other options.  Much depends on where you stay.  If it's someone you have met before, you could meet at some innocuous place and slip her a room key.  It's kind of tricky with co-workers around.  Perhaps finding someone with an incall and you could take an UBER there?

Many of my clients are visiting Vegas for work and have coworkers in the same hotel.
I suggest a specific designated area on the property and they pass me their second key.
I specifically mention the way to pass a key so there's never a problem.

I once met someone in the gift shop we both grabbed a bottle of water and go in line behind one another, not talking, and he slipped me the key . I waiting a few minutes after he left the store to head up

just wait for the lady in a major public area,  lobby,  by the gift shop,  near a restuarant.  just tell each other what you are wearing ...   when she arrives, just smile at each other and she will discreetly follow you ..  even go in the elevator together i bet you wont be the only ones in the elevator so nobody will notice.   dont fret.    

Usually the gift shop is a great place to discreetly pass a key off.

Thanks for the advice

I am happy to meet somewhere downstairs and have you pass a key to me or follow behind you so it doesn't look like we are walking together. I always dress business casual so I don't draw attention to myself if we are seen together downstairs.

Easiest way is to avoid this problem by booking your room at the Cosmopolitan. If not, meet in the gift shop, valet, or slot machines and give her the key and room number. You can also add her to the room as a guest and have her pick up at the front desk but most will try to avoid this.