I'd say it's up to the host...if they want a tight reign on the attendance (for whatever reason...vetting, game theme/structure, costs, etc...) or if they just want a loose shoot 'em up regardless of numbers...or whatever inbetween.
Regardless...they should (and do) put out their expectations in the game rules/notice...in the first post of the game thread.
It's plainly ignorant when players don't read through those notices and just expect to show up to do whatever they want. And at the end of it...the result is a mismatch of expectations. Examples:
- you end up with guys showing up in head to toe modern warfare multicam, complete with radios and helmets....at a WW2 re-enactment milsim. Real buzz killer for all involved.
- you end up with guys coming out to 24hr+ self sustaining events....with nothing but the boots on their feet and a ganola bar
- you end up with significant percentages of guys just simply not showing up for games....real game changer if you're planning a 60-90 person milsim....and only 30 people show
- you end up with guys showing up with nothing but a highcap to a real loadout, real cap game
...sure...there's always ways to deal with things...but why should a host have to deal with that? Why can't each player make the effort to understand what they're getting into...and certainly ask for clarification about ANYTHING well prior to committing to the game?
There are certainly lots of guys who "broker" their buddies/teammates attendance to games...I do that for some of my friends. There's nothing wrong with that. But if the game/host has put it out there that each person's name is required...then it's simply ignorant and disrespectful to not oblige, without being asked.
Anyways...it's not really a big issue or to-do. If the instance calls for it...the consequences of not reading a game thread will be clear when someone's +#s show up and suddenly find out that the +#'s can't participate because they're not on the roster. Someone's feelings might get hurt...but it won't be the host or any of the players that did sort themselves out...and that's ok.
Not brain surgery...
Perhaps to say that someone who does that is "too stupid to not be a risk to others" is harsh....perhaps another way to say it is, "someone who doesn't read and comply with the rules set out for an event is too ignorant to bother wasting any time with".
In the end it's simpler and less headaches to not have them participate.