For informal communication with other LAMMPS users you can join the ##lammps channel on libera.chat. It is an unofficial forum for discussing issues with other LAMMPS users. It's new as of 2019, and an experiment for the LAMMPS community to see if it proves useful.
For more ways to connect to the channel, take a look at: https://libera.chat/guides/connect
There is also a Slack channel for users who are developing code for LAMMPS, which can be joined by invitation, after sending a request email.