When I opened my mail I found an invitation to "connect on yahoo" from a name I recognized and took the steps to to "accept" the invitation. Now I see it´s just a way of sending adresses from your Yahoo address list out to other Yahoo subscribers. How can I unsubscribe from this?

Just ignore it -

I get those all the time; it's junkmail. I delete them without even opening.

Yes but I did open it and accepted by filling in some info. Now when I open my Yahoo mail that thing starts as the header. I want to unsubscribe. Ideas?

Look for link below the mail.. usually, there's a link where you can unsubscribe..