Take incrementing int32. Extend to 64 bits. Multiply by large prime number (e.g. fnv32 prime). Mod 1 million. Add 1 million. End up with random looking, 7 digits, nicely sequenced, 32 bit integers. Write an exhaustive test to verify.

When near 1 million users (yagni), reset sequence and do the same with 10 million (or one billion).

Doesn't solve the upside of 128-bit random numbers (ala uuid): the ability to generate remotely and expect no collision.