I don’t think there is any active place in lemmy for that, unfortunately.

This has been something I’ve been interested in and actually why I logged into this account today. I’ve gotten back into text erotic roleplay a little but the only places I could ever find enough people interested in this is on Discord servers and on Twitter. When I check the #nsfwrp hashtag on Mastodon I see a handful of posts from a handful of users, but it’s not really at any sort of critical mass to be able to find folks (unless you and one of the few people doing it end up being a good match).