LLMs could indeed address the first part, but not the second, of bringing the newcomers in via actual conversation with the older members. The only good solution I encountered to this is of having some (preferably not too experienced) member(s) actively take upon themselves the role of welcoming newcomers and answering their questions, whether that's in an official or unofficial capacity.

This to me is the real way through this "Eternal September", where in every "cohort" of newcomers, one or more choose to stay close to the doorway to welcome and guide the next cohort.