In high school you tell one person, they tell two, etc. Word of mouth. We didn't post flyers unless we wanted the cops to come also. We didn't make phone calls either.
On regular nights in high school there were two or three places to hang out. When the cops broke up one, we just went to the other. If you missed someone you talked to them the next day. Chances are they were doing something else.
In college in my Freshman year we would go with a group of girls to the fraternity house parties. They would let them in and kick us to the curb. Guess what I did after that? I pledged a fraternity. I was able to get my non fraternity friends in. There were probably 10 girls to every guy at the party. I don't know if things are different now but that is the way it was.
The social media apps, texting groups, etc, is what made it complicated.
The mistake people make is thinking things were easier. Things were less complicated and easier socially, but school was rough with typewriters, and research papers that involved constant digging at libraries for resources that might or might not be there.
When I went back to school years later I found it much easier. Then again, I was married and a heck of a lot less distracted!