If an external service only wants to include metadata about your journal’s articles - titles, authors, publication dates, etc. - there should be no problem allowing this. Such information is generally not protected by copyright and does not require copyright holders (i.e. the authors of the articles in your journal) to grant permission in order to reproduce it. See eScholarship’s open metadata policy for more information.
If an external service wants to reproduce the full text of your journal’s articles, whether that is permissible will depend on what your authors have agreed to when they published with you. The eScholarship sample author agreement we have been providing to journals since 2018 includes language covering this scenario; our earlier sample agreement did not. If your author agreement does not address inclusion in databases but does grant permission to publish articles with a Creative Commons (CC) license, the CC license may provide a path forward, and we would be happy to discuss that possibility with you.
If you are not sure whether authors granted you sufficient rights to relicense the articles in your journal, submit a ticket and we’ll see if we can help.