This is a tough one. If you have to broach this topic it means one of two things really. First, you aren’t really that close to this friend and there really shouldn’t be a conversation. Or secondly, that you are having a smaller wedding and your friend is mildly clingy and this requires a delicate conversation.
The best tactic to get around this situation is justification. You obviously have your reasons for choosing a wedding party and there are signals that you can relay to your friend when having this conversation to make them feel better about not being “chosen.” Finding out you are not as close to someone as you thought you were can be a tough pill to swallow, but with a little tact you can make that pill a Flinstone’s Vitamin Jelly.
While letting them know how close you are to this person you can also signify the familial like bonds you have with those who are in your wedding party. Detailing the length of your friendships with those chosen or the significant life moments that have cemented your friendships with the others will help clarify that while you are still great friends, there are justifications for those you chose.
This is the best advice I have ever gotten though, so write this down – Control+C followed by Control+V will work as well – Bad news travels just as fast as good news. People often like to beat around the bush or not address the elephant in the room to the point where things get uncomfortable. Just rip the band aid off and let them know right away and that way you will save all the awkward exchanges and get over this sticky situation.