Great idea, Definitely time for a reminder, but probably won't stop them from moving in whoever they think you won't notice. Might be good or bad depending on who moves in.
Case in point: Rented to two 30 something brothers in mid-September. Nice guys, good income, with extended family who came over each Sunday for 'football watching'. Great people, quiet, pay on time every time, no complaints from them, following all parking rules, no noise.
So MOM asked me if she could stay as a guest for 2 weeks because of Covid quarantine of her cousin. (nobody had it, just precaution). So after 2 weeks, she was still here (cleaning, fixing, nesting, shopping for 'the boys', etc.) Nice lady, even gave me a Xmas present. Wow! January, she was still here so I was about to say something and then noticed she had come home with a car loaded with personal items. Loaded loaded loaded. I jokingly said, "looks like you are moving in" and she said, "Oh, my son was supposed to tell you." Well he didn't and she is still here going on March now.
Still no problem AND because my sister told me about charging a 'guest fee', which I was doing and making an extra $150 per month to have Mom here (short term I thought), the extra money was nice and it covered the extra water use. So after Mom moved in, and not letting me know, I told the son that per our Rental Agreement, there would be an extra charge, for the extra person and she needed to be added to the Agreement. I purposely didn't specify the amount in the Rental Agreement so I could charge what seemed appropriate to the circumstances.
I upped the charge to $300 a month and added Mom as an Occupant not a Tenant. She's trying to buy a house (sold her old one) and was living with the cousin. I was told this from day one and expect she will move if and when she finds a place. Hopefully this will be long-term and the extra cash is nice and the 'family' is a happy crew.
Sometimes it works out. Usually it doesn't. Last group I had kept 'bringing in strays' which really caused me a ton of grief. Made more work for me to hound them for extra 'water' money. Last one before they all moved out was a convicted felon (loads of charges). Not a nice time. --68.8.xx.xx