We've been using a separate tool for newsletters, but I'd like to move those subscriptions into Civi - basically, add users to a mailing group in bulk. There are several thousand of them. I'd prefer not to send them all an opt-in link since they already opted in to the existing newsletter.
What's the best way to accomplish this? All the contacts are already in Civi. I can grab a list of the contact IDs, so I think I could probably handle this just on the database side, but I feel like using the API or SearchKit might be a better approach... I just am not sure how to actually do that.