Your Tier is based on the Tier Points you accumulate during a calendar year:
Bronze: 0 - 4,999
Silver: 5,000 - 24,999
Gold: 25,000 - 49,999
Platinum: 50,000 - 99,999
Diamond: 100,000+ (by invitation only)
You are automatically enrolled into Bronze once you successfully sign up on Trove.