I just configured Indie Hackers to require an invite code for signup.
My primary motivation is to fight spam. Hundreds of companies and individuals create dozens of accounts every day for the express purpose of posting spam. Playing whack-a-mole is annoying and feels un-winnable, but an invite system should stop new spammers from joining instantaneously (or allow us to track down the spammers who are inviting them).
There are secondary benefits, too. I'm only granting invite codes to product owners who are actively making useful posts to the community. Chances are, the people they'll invite to Indie Hackers will be more helpful than average.
(Note that earning invite codes is an automatic process that happens daily.)
I admit that it'll also be cool to see people asking for and sharing invite codes on other websites, like Twitter.
The downsides: Making IH more exclusive when I've always erred on the side of radical inclusivity, and of course, decreasing signup rates.
I'm not so worried about the signup rate. Growth is a function of both signups and retention, and I suspect this change may do more to boost retention (by improving site quality) in the long run than it does to hamper new signups.
As for timelines, I plan to keep things invite-only for at least a couple of weeks to see how things shake out and to give us time to discourage spammers. If it's a disaster, I can easily reopen signups. And if it's great, I can always keep the new system as long as I want, or even indefinitely.
Please let me know your thoughts below!