I did that a few months back when there was a panic around Reddit and got in quickly, it might have helped that I posted links to my HN profile (at least they know if I get an invite I'll post something ;-) and my Mastodon and also a few sentences about why I was interested. I've had no luck though at getting into any Lemmy, however.
I have heard though from people who've sent multiple email messages to Tildes over weeks and haven't gotten in. It could be they are busy, feel full, etc. But I think talking about what you can for them is better than what they can do for you and this is true about job applications and so many things.
Unfortunately I've given out all my invites and haven't gotten any more yet.
