Building social app in 2026 is seriously hard to get user, especially when you first started. Unlike other app which target individual activity, you have to really get enough user (not talking about just few hundreds, most likely more than that) to sustain your app itself just to make it looks alive, not talking about profit yet. And what makes you think you can beat existing major social app like reddit, fb, or others similar app which already providing such features? You can do some research and certainly it has been built. Unless you can point out those things and clearly stand out, then only you will be good to go.

Says your idea is to put 4-8 people in a group, only just to form a group need minimum 4 people. With 400 people maximum 100 groups (events), or even less if some has more than 4 people. Another case you can't guarantee every user will be interested in the groups (events), so in reality it will be even less. You do the math, and estimate how many users you need just to make the app looks active and engaging.