Over the last 5 or 6 months new members have joined our fairly small team in a large company. Previously there were just two of us on the team. Everyone on the team gets along pretty well. Personally I get along well with the new joiners. During breaks between coding we'll often laugh joke around about the recent ongoings in the office.
The catch is recently I found out that the new members have been regularly hanging out after work. Moreover when my manager tries to organize team events, most of the time my other coworkers say they can't come. There is one other person in their group that's in a closely related team.
I know that they have every right to hang out with who they want to, but I still can't help but feel crappy because at this point I thought these coworkers were friends. I'm considering moving to a different team, but up until recently I told my manager I was happy in the current team and had no plans to move. I also understand my manager does not and should not have any responsibility regarding who my coworkers choose to hang out with. However I also feel it's wrong to lie to my manager about the reason for leaving, especially since I had recently told him nothing was bothering me (which, at the time, nothing was bothering me). Or since it is outside his purview, should I just come up with some generic reason?