Yes, I would check them out and see if it's worth any effort.

I think there is at least one site that does invitation only in the niche I'm promoting. I know for sure they have an affialiate program, but they don't have a webmaster link on their site. So a while ago I send them a few emails about joining their program but never had any response and stopped trying to promote them.

If in future they would approach me about promoting their site I would decline it. If they are not responding to polite emails, they are surely not someone I would want to work with or promoting them.

So it all depends on who they are and how they work. I'm not automatically flattered when I receive an invitation and not all 'invitation only' things are as good or as important as they probably think they are.