You can follow a group by tracking the topic, not the words.
Group conversation moves fast because turns are short, and the trick is to stop chasing every sentence.
One to one conversation is generous. The other person waits, adjusts, checks you are with them. A group does none of that. Turns are short, people talk over each other, and the subject can move three times in a minute. Trying to decode every sentence is the natural response and it is the thing that loses you. You process one line while two more go past, and then you are behind for good.
Listen for the topic instead. Who or what are they talking about, and is the feeling positive or negative. Hold those two things and let the individual words wash through. You will be surprised how much of the sense arrives anyway, from tone, from laughter, from who looks at whom. Staying oriented is worth far more than catching each phrase, because orientation is what lets you rejoin.
And ask when the subject changes, not later. Sorry, who are we talking about. What is that, the place near the station. These questions are welcome in groups. They signal interest, and someone will happily fill you in. Then take the next short gap and say something small. One sentence puts you inside the conversation, and being inside makes the listening easier straight away.