I'm only very new to this poly thing, and I am in an emotional triad, functional vee. I LOVE your description of 'invitation.' Invitation has been a huge thing for me in my life, as I seem to be surrounded by people (family and friends) who feel that invitation is assumed. Whereas I do not assume that I've been invited unless I've actually been invited.
My triad took 8 years to happen, and it's not what any of us were actively seeking. I chose people who resolutely, stubbornly, maddeningly, with steadfast love, refuse to submit to my agendas.
I actually think Current bf is more inclusionary than even I, and definitely more than First bf. He talks to anyone and everyone, he invites everyone to anything. I've always credited him for allowing the three of us to rebuild our friendship after I broke up with First bf.
I believe (now looking in retrospect), that my behaviour with First bf has constantly been of invitation. And loving. Just 'i love you, as a friend, no demands' and including him in our lives in such ways as were possible for all of us. And that consistent response is what led us here.
I've been uncomfortable seeing folks use the term 'bringing someone in.' When I was young (a hot bi babe), a couple hit on me as people were filing out of the (gay) bar one night. It was incredibly creepy, and the feeling lingers, as much as the fantasy appeals.