Inviting people to your event is a quick process, available on all Plans to all users. Here's a guide on how you can invite folks step-by-step.
Click on the Invite button for your event
Find the event you're looking to invite people to. Next, click the "..." button (it will be displayed as "More" on mobile devices). Click "Invite users / channels". You can select a user or a channel from the dropdown menus. By inviting a channel, Eventbot will post the event inside that channel (it won't directly invite every user in that channel).
Some notes about inviting
You are only able to invite someone once. That person will receive their invite and no other invitations will be sent for the same event (this is to prevent duplicative invites).
Feel free to invite as many people as you like! If someone RSVPs to the event, it will be visible on the Details view of the event. They can also repost the event to other channels.