If you are interested in optimizing your experience gain, you may want to refrain from getting a 6th person in the group unless they will significantly increase the rate at which you are killing.
I don't really agree with this analysis, or at least with the wording of it. Even if there were no group bonuses at all, if every group member contributes equally to the long term kill rate (in the case of some classes, very indirectly), you're still better off with as many toons as you can fit into a group, if nothing else for the company and the flexibility on targets it gives you. The group bonus system is supposed to offset concerns about groupmates who are relative slackers, or general inefficiencies that arise from group coordination and "paying" for abilities that often go unused for long stretches (e.g. rez). I guess it pays for the extra setup effort putting a group together as well.
Generally speaking, with bonuses in mind, adding group members who aren't total slackers is always a win at any group size. There are anomalies at the 3-4 and 5-6 boundaries, but I'd word them like this:
If you've got a group of size 3, there's absolutely no reason not to add a 4th toon to the group. Even if the 4th toon is AFK the whole time, the original 3 will make slightly faster XP than they did without him, and the 4th guy gets a bunch of 'free XP' to boot. If the 4th guy is contributing at all, it's an even bigger win.
If you add a 6th man to a 5-man group, and the 6th man doesn't 'pull his own weight' relative to the average contribution of the other 5, it's a very slight net loss. If they're contributing reasonably, nothing really changes.
There's no need to actively discourage people from filling up that last group slot with other human beings. And unless the 6th is AFK, odds are good he's on par with the other 5's average contribution.