There can be several different reasons as to why some people are not receiving the invitation through email.
Did you enter the right email address? A letter or character can be easy to misspell.
A spam filter in your email program or Internet provider may have identified e-mails delivered by Spond as spam. Check your spam filter to see if you can find the invitation there, and then indicate that messages from Spond are wanted.
Do you use an e-mail account that automatically forward emails to another email account? In some circumstances it is possible that the e-mail server has blocked the forwarding feature. Make sure that the person that sends out the invitations uses the right e-mail address (without the forwarding feature).
PS! It is rare, however sometimes emails use minutes or even hours to be delivered.