If you are a site admin, check out Site Admin Menu, Users, Builk users ... set filter ... check the list ... then if OK, select.
There will be an option to bulk email a message.
Caution: if using Moodle's default phpMailer and not an SMTP relay that will allow hundreds of messages, you could possibly get your Moodle listed in a blackhole/list as a spammer box ...
OR if using SMTP relay, overtax the SMTP server. The latter will queue up messages and attempt to send them via adhoc_task at next run of cron job/scheduled task.
'SoS', Ken