Can I send an email to users of the helpdesk? Or to a certain group?

You are able to send an email to all users of the helpdesk, or to a specific groups of users. To do that go to Admin Settings > Email Users. Here you can click on +new mailing, and choose if you want to send an email to all users or to a specific group.

Only agents with manager permissions and above have access to this feature. The email will come from the agent setting up the mailing. Note that suspended users will also receive the email. After that email will be saved in the helpdesk, along with the number of recipients. 
See other articles in Email FAQs