The game I've been playing started with 4 players. One of them had to take a hiatus for schoolwork, but they'll be coming back very soon. We recently filled that player's spot with a new player, which will be 5 when the player on hiatus returns. Now my players would like to introduce yet another player to my game. I flattered by the suggestions, though I'm nervous about the now larger number of players and what I need to do to adapt the game to a larger audience. What do I need to keep in mind before allowing a new player into the game? What are the implications and effects of 6 players on a game system initially designed for 4-5?
System-wise, I don't think it's a problem. You might have to adjust encounter CRs for published adventures a little, but that's about it.
But GM-load-wise, and combat-length-wise, it can be a drag.
I used to think that I needed at least five or six players for a good game. Four didn't seem like enough. But I've come around to the discovery that four players is really my ideal, and five is pretty much my max.
If you're into the roleplaying and are concerned about spotlight time for each character, having sub-plots or such for individual characters, etc... every player you add complicates your job as a GM. You have more sub-plots to create and keep track of, more individual scenes to juggle.
Then more players means cutting into each others time more. From more time between turns in combat to more time waiting for time in the roleplaying spotlight... the GM has to work harder to keep everyone involved and interested because they're waiting longer for the bits of the game that interest them the most.
Not saying don't do it, but you have to ask yourself if you're creating more work than you want to handle, and ultimately, will the other players' enjoyment of the game be increased or decreased by adding another player. If that new player is someone they really like and want to play with, especially if they're a great roleplayer who will bring a lot to the table, they can be a positive change despite the extra player load. (And a good player can reduce the GM's work load, or at least make it so much fun it seems like less work.)
There are more players in the room, which (apart from the obvious logistic problems of accomodating them around the table) means there's one more person who can get distracted or start a side conversation. In my personal experience large groups are less productive because most of the time is spent talking to each other.
The first and easily spotted consequence of having a new player is that adventuring takes longer. There's one more person in the initiative order when fighting and he needs some screen time too out of combat. This in turn means the other players could get screen time less often, which might be a problem to someone.
Combat takes longer to plan, because you need to fight more people at once. There's also more characters focusing fire on a single enemy, which makes it essential to buff the encounters so that the enemies are more resilient or numerous.
Combat rooms might need some enlarging, to let all the characters (PCs and NPCs along) some way to get to their targets, especially if the party is melee intensive.