1. Go to evenium.com
2. Click on My Events
3. Click the name of the event under the I Organize section
4. Go to the Participants tab located on the purple bar
5. Filter your participants list as desired and select participants to whom you want to send an invitation by checking the box left of their name. You can easily select all currently filtered participants by checking the box at the top of the leftmost column. 
6. Click Emails. That'll take you to a page where you can select and edit the email you'd like to send. 
7. In the dropdown menu, select the Invitation email. 
8. You'll see under Recipients your currently selected group of participants and other selection options. When you're ready to send the email, hit Send on the top right!