You can use default groups already existing in the system or manually filter and select participants from your participants list.
Default groups are handy when you want to send an email to all your confirmed guests, those who didn't answer your invitation yet, people you just imported and you need to invite,...
Manually filtering and selecting participants from your participants list allows you to send emails to very specific groups of attendees.
Using default groups:
Go to the back office of your event in Configuration -> Invitations & Emails -> Edit invitations & Emails. Under the Recipients section, you'll see a dropdown menu from which you can select any of the default groups of participants.
From your participants list:
Go to the back office of your event on the Participants page. Filter your participants list with your desired criteria, then select some or all filtered attendees by checking the box left of their name. Note you can easily select all currently filtered participants by checking the box at the top of the leftmost column.