To whom it may concern,
Remember that Aikido is not only physical, but it is mental and spiritual as well. Well, as well as with most martial arts.
I would say that I know how you feel because I was actually one of those people that you talked about, the one that trains within my own "group". I have learned a way past this, but sometimes old habits die hard.
My way of doing it is just wait until everybody is partnered up and I'll be partnered with whomever is left. I do expect the juniors to be more active and seek the seniors instead of the other way around. If someone come to me and asked me to be a training partner, I would gladly accept. If I see someone sitting down with nothing to do, I approach them and ask them if they want to train. Please enlighten me if I'm wrong.
I know how you feel because I'm one of those people that created those type of environment and I realize the impact that it creates. I train with people that I started off training with. I do have my own little "group". I do still have that, but at the same time, I am now more open, more outgoing, instead of just hiding in that group of people. Training not with the same group of people over and over actually made it more fun.
In my defense of creating my own little group, it is because I see the others doing it. I know you think that's a poor excuse and it actually is. It is not until I train around other dojos and meet new people, forcing me to traing without my group, that I have learn the enjoyment of training with others.
One has to be more active seeking out training partners. Force them to be your partner, cutting off their "usual" partner. Trust me, you're doing them and yourself a favor.
I do still have one problem, training with people that I don't like. Some friends of mine could actually do this. They were really sincere with the "disliked" partner also, I could really see it in their faces. Regardless of their dislike to those people, they could be 100% sincere at that moment in time. Not like smiling on the face but with a knife in the heart. Not being a hypocrite. I could tell the difference between the two. What happens after that is a different story, but this is something that I need to learn to do.