Im reading the information as this ("Invite your friends to play Raid and win rewards as they play"), that you dont get the sacred shards for the invite itself. You get them as soon as the ones you invited gain levels.

Aedzy is correct. There are level thresholds your friends have to hit for you to get the rewards- you get sacred shards once they reach level 50 (and a bonus sacred shard once all 3 friends hit 50).