Custom scenario invitations

Past few days a couple of times I received invitations from players who weren't added as friends or in my last 25 played.

Was wondering how is it possible for them to send an invite in this case. Also when I accepted one of them it gave a warning that I have to download the scenario and only do so if I trust it.

Are these genuine invites or someone trying to get me download some virus?

Also if these are genuine invites what these custom scenario like are they interesting to play?