My experience at training in multiple disciplines, striking, grappling, and weapons, is that if you are unarmed then it is very easy for opponents to close to clinch range and, most of the time, the opponent will not absorb sufficient damage from striking during his entry to stop him. Once in the clinch, the fighter with superior clinch-work/thrower will decide whether or not the fight will "go to the ground." Once on the ground the fighter with superior ground-work skills will decide whether or not the fight will continue on the ground.
These things are simply "mechanical" facts. Add in the fact that "wrastling around" seems to come natural to humans (not actually being "good" at it, just the inclination), and you have a brew custom made for a large percentage of fights hitting the floor.
Beyond that, you get into cultural artifacts. Essentially, in order for most fights to not hit the floor there has to be some sort of mutual consent, either spoken or unspoken. In some cultures, historically, getting into "wrastling" during a "fight 'twixt men" was considered unmanly. You just didn't do it. In other cultures, just the opposite was true: "men" settled their differences by wrestling and no one used striking, weapons, or "maiming" techniques.
As to your friends' specific concerns that they wouldn't "want" to be on the floor... well duh. They don't know what they're doing there. I wouldn't want to ground-fight either if I didn't know jack about it. That said, if they don't know anything about it, it's freaking hard to prevent it from happening.
Sure, maybe the opponent "brought friends" but may be you did too. Maybe your opponent is busy thinking "I hope he didn't bring friends..."
Something else to consider: Due to the influence of MMA, grappling is in something of an ascendancy right now. Everyone is thinking "empty hand fighting includes ground grappling." ...Especially the "young punks" and "street toughs."
Besides superior grappling skills and "social structures," what prevents opponents from clinching and grappling? Weapons.
So, if you're not prepared to learn grappling then you'd better be prepared to knife someone trying to do a take-down on you.
In conclusion, if a fighter wants to "be prepared" then he needs to study grappling 'cuz this "anti-grappling" stuff doesn't work. If a person wants to know how to effectively counter grappling, he needs to understand grappling. If a fighter wants to effectively counter blade-work, he needs to study blade-work. If a fighter wants to effectively counter firearms, then he needs to study firearms.
</rant>
Hope this helps.
Peace favor your sword,
Kirk