This is a dodge.

Any non-strawman model of individualism has room for…voluntary engagement with other humans.  No one – actually no one – is saying “individualism, therefore you’re not allowed to form communities of interest.”  Or “individualism, therefore you’re not allowed to find emotional satisfaction in closeness to other people.”  Or “individualism, therefore you’re not allowed to be invested in your awesome summer camp.” 

(OK, hypothetically, you might just be taking a shot at the sort of small-minded hyperconformist morons who look at anything other than nuclear-family-centric suburban anomie and say OMG THAT IS A CULT!.  But if so, uh, that’s a pretty damn soft target and no one gives a shit.) 

The individualist society is a society of voluntary communities of interest.

But voluntary communities of interest do have their limits.  Most importantly on the macro scale, your kids aren’t likely to stick around to be part of them.  Also, well, even when people love each other a lot, they do in fact change and get into fights and generally make decisions that put a strain on community cohesion.  (I am very familiar with the internal dynamics of Awesome Voluntary Communities Full of Love Where Everyone Is Super Invested.  I have seen them self-destroy hideously.  I’d be surprised if this weren’t also true of you.) 

And these limits…have consequences.  A community where anyone could just up and leave, if he gets sufficiently steamed – even if you’re pretty sure he won’t – is a community where you’re probably going to be wary about just handing over lots of your resources.  A community that isn’t likely to contain your kids is a community that isn’t going to absorb the entirety of your attention and your wealth.  Etc. 

You can get around those limits.  Most human societies do.  You get around them with coercive power, of the temporal and social varieties.  You get around them by ensuring that people actually can’t leave, or that they will pay a huge penalty if they do.  You get around them by ensuring that your kids are vastly more comfortable in the community than they can ever be outside it. 

(The “constant fanatical enthusiasm” thing is kind of a strawman in and of itself, on the part of your interlocutors.  No one seriously thinks that you can run a tribal society on that engine, which is why no one has ever seriously tried.  But many dyed-in-the-wool individualists aren’t great at grappling with social structures like “if you walk out you will have zero money and zero skills and no one you know will ever speak to you again.”  Which is funny, because presumably most of them grew up in modern Western nuclear families, which are mini-tribes that employ all the usual coercive tactics.) 

There are concrete advantages to tribalism, although I am firmly convinced that they are really really really really not worth it.  But if you want to defend tribalism, start by talking about tribalism, not about some kind of magic inhuman society that has all the freedoms of atomic individualism but it doesn’t matter because you can count on no one ever using them.