Go to Attendees -> Invite/Import.
The people you import here will be sent the "Invite"-email you can find this under "messages" -> "invites"
Note that this is a paid feature so you can not send your invites through us if you have the "Free" version.
If you want to invite a group of people at once you can use the "Group" alternative. Then you get a box you can copy-paste a list of names and addresses into.
Just make sure the name is first and the email second in the list you copy from.