Thank you. It's clear to me now. The invitees created the account and logged in the web version of the app. They didn't installed the app. I'll tell them to do so. I really want to try the premium account in order to decide if I upgrade my free account. Once again thank you so much for your kind answer. Jero.
I wanted to try the premium features so I've used the "invite a friend" feature to earn 10 points and try it for one month. I've seen some threads in which the people are talking about they have to open a ticket to get these points. And this seems to be my case as well. Maybe it's just it takes a while to update the points or some additional actions are needed. BTW, I cannot see where a ticket can be opened to track this issue (if it's an issue). Thanks for the help :-D Jero.