“Add to Campaign” Button is not visible in Lightning Experience for Standard users unless 'Account and Contract' is set to Public Read/Write under Org sharing settings .Once the button is visible, Standard Users can add the campaigns to the contact/person account. In Classic, the visibility on 'Add to Campaigns' button is not effected by 'Account and Contract' sharing setting".
Set "Account and Contract" to Public Read and Write under Org sharing settings to make “Add to Campaign” Button visible for Standard Users.
Reference :
Enable Accounts as Campaign Members
004576438

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