If you have been given access to more than one WeTrack account, once you login to one of those accounts, you are also logged into the others*.
If you login through wetrack.com you will be presented with a list of the accounts you have access to. Simply click on any account to access it:
If you are already logged in to an account, you can quickly switch to other accounts by clicking Switch Account below the Account Logo in the left hand menu. This will open the Account Switcher, from which you can select the account you want to access.
*NOTE: You will only be logged into WeTrack accounts for which you use the same email address to login. If you have different email addresses on different accounts, you will need to login to these separately.