My concerns are any associations these guys may have and I'm thinking of a rule where there would be no visitors allowed. 

I'm viewing the 17 YOs as long-term visitors.

It was once a beautiful city supposedly but these days it's basically a shanty-town for the most part and widely considered a no-go zone for tourists to visit or expats to live in.

(Example: sharing the kitchen, having too many visitors)3.

The lease stipulates no visitor can stay more than 48 hours without being added to the lease.

I have the time, I'm good with people and I would really enjoy helping visitors with their stay and getting the most out of it.The trouble is that it would of course have to be close enough to where we are in South CA, or I wouldn't be able to go and help / check in / work on it.Big Bear Lake is one I've seen but it's just a bit too far to be practical.

Finding out that the speakers/presenters and/or other visitors are totally clueless morons who have never closed a real deal in their life, live on the second floor of their mom's house, and are up to their eyeballs in debt yet they're trying to pretend their REI gods and know what they're talking about.

If she stays...) but merely as a "not-horrible" tenant.I won't go into too much detail but I truly am on the fence about this tenant as sometimes, she makes ridiculous demands (Ex: "please swap out my perfectly good bathroom tub for a deeper, more luxurious tub at your expense simply because I want it")She also seems to be oblivious to the fact that she is the tenant in the building that creates the most risk for us, brings the most visitors to the building (with all the slip&fall risk they bring), creates the most noise in the building, uses the most water & sewer (which WE pay for) while paying the least in rent.

The heroin addict visitor like @Joseph Brickler mentions is a problem with no real solution and conducting 'illegal activity' is difficult to prove.I have found them to need some basic education.  

The more 5 star reviews you get the more return visitors you can direct to the direct booking site for a start.