In a given advertising campaign, you can specify a single main advertiser and an assistant, as well as several additional advertisers whom you also need to grant access to the campaign and its reports.
You can add additional users in the campaign settings. On the campaigns page, click next to the desired campaign. Click the + Add advertiser or + Add assistant link and select the user account from the list.
On the Summary tab, you can view the list of additional assistants and advertisers for the campaign.