Adding any number of extra spots in a group for dps doesn't make a game inherently more fun, first of all. Not to mention a game balanced for 8 man groups means content is tuned for, what, 5 dps spots, which is what everyone will aim for. A tank, a healer, maybe a support or second tank/healer, and then stacked dps. Meaning if you and two friends are online you'll be fucked if you want to do anything. Keeping groups 5 or 6 is a pretty good balance between having 'enough' spots for friends (which is an idiotic, unquantifiable stance anyhow), and keeping content balanced so smaller groups are still viable. The bigger the group, the more the shift in dps requirements raise, and at an exponential rate that screws over smaller groups.
Here's an old post I found from some board I've never heard of:
Shadowpuma View topic - Vanguard : saga of heros group size
Vanguard : saga of heros group size
Thankfully, fairly overwhelmingly if I recall, we let him know that no, it didn't make sense.
I still think 5 or 6 is the sweet spot. 5 is good for games that are generally going to be more forgiving and an easier world to navigate. 6 better for games that are going to be generally tougher and less forgiving. Group sizes of 8 and you're suddenly having to design 'casual content' for small groups instead of giving skilled players a shot at meaningful content. And I'm not personally a fan of that, as it's similar to me as designing solo content (instead of just letting some classes and some players work out ways to succeed solo, like kiting, which wasn't because content was created to solo, but because players figured out ways to survive the content that simply existed in the context of the game).
So, yeah, I think anything about 6 turns into a shitfest, both in terms of it starting to feel more like a Diablo zerg, and in the fact it basically pushed the balance of power such that you're not making the game more fun because your theoretical 2/3 extra dps friends have to sit out, but making it less fun because smaller groups either don't have a chance to do anything until they add more (dps) or 'casual' content has to be created, and then enforced, or you have people steamrolling the 'easy' content to rush to max level, ala the solo-to-max syndrome that exists when a games balance of power is pushed too low or solo/casual content is specifically created.
One of the few things I'm still on the fence about in Vanguard is the dot system. I basically don't think 1 dots should have ever existed, because a minion is a fucking minion and doesn't need to be classified by difficulty rating. 2 dots were almost always too easy, and practically had a neon sign explaining they were catering to the solo-friendly mindset WoW created (and it's no surprise the majority of quests seemed to involved 2 dots, which is why people just grinded mindless, easy quests all damn day while dungeons were empty).
Basically, the outside world should have been 3 dots, with the occasional 4 dot named. And dungeons should have been 4 dot with the occasional 5 dot named. And raids should have been 5 dot with the occasional 6 dot named (where there 6 dots, I can't remember? hah). And how hard is that to remember based on the context of what you're killing that it needed advertised? As a player you should know what to expect by where you're fighting enough to not needing a dot or champion or elite indication system.
The dot system seemed to be a relic of the 8 man grouping idea, and the notion they needed to create mobs and content for every possible type of gamer. Hopefully they've gone away from that are are just going to create a world that is generally tough, and gets tougher the deeper you go into areas we're all smart enough to understand should contextually be tougher.
But, yeah, I generally liked Vanguards solution to grouping woes; the main one being classes that were interesting and fun and solved a lot of grouping related problems like not enough healers, not enough tanks, and all your lame ass friends wanting to play mindless dps with no support. Vanguard had less dps-class over-population than most games I've played, and the dps classes that did exist had far more widespread function and support than just mindless dps. Things I'm okay with.
That said, more support and wider function than just straight dps isn't an excuse for ability/skill/spell bloat.
