Find all group of neighbors with a constraint weight
Your problem is the same as listing all cliques in a graph.

Given your weight matrix, construct a graph in which two vertices are connected if the weight between them is below the threshold. A clique in this graph is the same as the sets of vertices you are interested in.

A graph could contain exponentially many cliques - even exponentially many maximum cliques (consider for example $n/3$ paths of edge-length 2). So in general you will need exponential time to list all cliques.