Simply open the exhibitor's profile in the app, then send them a message directly from there. You can also add the exhibitor to your favorites so you can find them easily. It is not currently possible to network with other visitors via the Paysalia/Rocalia app. You cannot send them messages either.
Go to the exhibitor's profile in the app and choose a time slot that suits you.
Appointments can be scheduled in advance or during the show, but will only take place on site.