Assuming it is a clan-member joining, kicks of non-clan members are determined solely on playtime. The player with the least amount of time in the server is kicked. Last one in, first one out. Like jobs. When a player is spectating his playtime doesnt increase but he is still counted as a clan member. Playtime carries over to the next round.
Beyond that:
I spent a lot of time and worked on several different methods to include additional weights even getting to the point where it counted k/d ratios. Whether or not a particluar weight was fair or had the right priority depended on who you asked. Snipers have low scores but usually high k/d. Add time played to that. Then it becomes a judgement call on how to balance it all compared to specops. That class usually has higher scores and closer k/d. Again add playtime. And finally which is more important? Score or playtime? Before you ask, 'majority rules' doesnt work. So the decision of sticking to time played was between myself and Rugger.