It’s tricky because Lemmy doesn’t support following people directly.

I think you can @ people on Mastodon/Misskey/Firefish by @user@instance.tld and they will see it, and I assume if you reply to a post or comment made by them (since they can post/comment here) they can interact with it normally.

No idea if using hashtags on Lemmy will populate people’s Mastodon feed with those posts, but I assume… not?