You must be a Company Page administrator in order to add other administrators.
Note: You must visit LinkedIn.com from your desktop and go to the Company or Showcase Page to make admin changes.
- Move your cursor over Interests at the top of your homepage and select Companies.
- Type your company name into the Search for Companies box and select your company name from the list.
- Click the blue Edit button in the upper right.
- Scroll down the Overview page and find the Company Pages Admins section on the left. This is below the Company Description and Default Language fields.
- Under Designated Admins, start typing the name of a connection that you want to be an admin. You must be connected to the new admins through LinkedIn before you can add them.
- Note: The maximum number of admins for a Company Page is 50.
Steps taken from: https://help.linkedin.com/app/answers/detail/a_id/37594