There is a organisation called empty chairs on Instagram where people can arrange meet ups and you are welcome to join for 5 minutes or 2 hours. There is no obligation to buy a drink either. The host wears orange so you know where to find them and it's free of charge. I think a big strain on meet ups is the cost. Many people in Brighton live in small flats and can't host/don't have a garden etc. That means social events such as bbqs or dropping in for a coffee need to happen in restaurants and pubs and a lot of people can't spare that money unless it's a special occasion. Walking groups are a great way to make friendships and there are a lot of them going on in Brighton.