Links with additional information and links for further reference:
IUCr Commission on Mathematical and Theoretical Crystallography:
https://www.crystallography.fr/mathcryst/
IUCr Commission on Crystallographic Computing:
https://www.iucr.org/resources/commissions/crystallographic-computing
IUCr Commission on Quantum Crystallography:
https://www.iucr.org/resources/commissions/csmd
Most IUCr Commissions have their e-mail lists that, in most cases, are open to joining. For example, relevant announcements and advertisements of talks and events in mathematical and theoretical Crystallography are announced frequently on the list of the MaThCryst group: https://www.crystallography.fr/mathcryst/mailinglist.php
Psi-k Network and E-mail list (mostly electronic structure calculations):
https://psi-k.net
CECAM (Centre Européen de Calcul Atomique et Moléculaire):
https://www.cecam.org