If you need to join multiple fleets, you will need to create separate Meter accounts with different email addresses.
How to Drive for More Than One Fleet
Create a second Meter account - You will need a different email address for each account.
Sign in separately - When switching between fleets, log out of one account and log in with the other.
Manage trips and payments - Since each account is separate, your trips, earnings, and settings won't transfer between them.