When I ran more impromptu or loosley scheduled stuff I used a site called Doodle. I entered when I could run and potental participants could mark available etc on them. Worked well enough. Would have loved something where sessions could be suggested but that never really became an issue. You could do that with a google calendar. Someone adds a potential session, sends invites to everyone who then can mark their availability. Should work. You could also look into workplace meeting or planning apps.

Nowadays I never ever, not even if bacon flies, reschedule. Cancel sure but never reschedule. I don't have the time and most of those I play with don't have the time. To keep things running I recruit to five, have an ideal group size of four and run if three or more are available. Barely ever any cancellations. Only ones if I'm not available or if there is SIGNIFICANT plot development.