You will receive an email invite for each company you are being added to. Please follow these instructions to ensure you have them all added.
When you receive the 1st email, click the link in it and it will prompt you to create a password, then will log you in. When it does, BEFORE you open the next email invite BE SURE to Logout, by clicking the 
icon in top right, then clicking logout.
After you logout, open the next email and click the link and this will add the 2nd company to your profile. THEN LOGOUT AGAIN.
Then click the next email... etc.
If you don't logout each time before clicking, it will not add that company to your dropdown list.