Having a lot of friends but no actual friend group in high school sucks. How do I change this?

I’d say I have 10-15 “close friends”. Despite this, I don’t have any 1 “group”. This means that while I can generally find people to talk to during school, I almost never get invited to hangouts, birthdays, parties, events, concerts etc etc. This also means I’m not actively involved in larger group, discussions or conversations. If someone asked me who my best friends were, it’d be almost impossible for me to narrow it down. Even then, I know for a fact that people I’d name my closest friends all have 4-5 people they’d rather invite to places etc before even considering me. It sucks, especially in High School when it can be several months before I’m invited to a friend’s house (which is usually for a group project anyway), or when I’m wandering alone during the lunch break. Is there any way to fix this or should I focus on my grades and future?