Can I invite more people after I send the invite out?

Yes, you'll be able to return to your invite any time after sending to invite more guests.

To do this, first, click the Manage Invitation button to the right of the invitation on your dashboard. Then, on the next screen, click on the Add More Guests button.
Here, you will be able to enter your guests' email addresses or phone numbers and then proceed to the Send page to send the invite out. You can read more about the different ways to add guests here.

To invite more guests via the mobile app, first, tap on the Dashboard tab and select your invitation.
Next, tap on the Manage guests button and then the Add Guests button.
From here, tap the red "+" button. On the next page select how you'd like to add the guest, enter in the guest's name and then tap on the Add to guest list button at the bottom of the page.
Note: This will send the invitation just the newly added guests. It won't resend to guests who have already been sent the invitation.