As LinkedIn notes in its Recruiter Mail Policy, you should not use InMail for mass mailings, invitations to various events, and for marketing campaigns. Recruiters usually do not launch marketing campaigns, but it is better to pay attention to the point of mass mailings and not abuse it. The best option is to wait for the candidate to confirm the addition and start a dialogue. Even if you write to many candidates and have some kind of template, think about how to adjust it so that it can be adapted to each recipient, and there is no feeling that this is just a mass mailing.