If lemmy created a community the same name as the username when someone signs up. You could follow a lemmy user and then when your on mastodon (once we’re federated with mastodon) you could see their self posts in their community as a mastodon post and vice versa.
You could do that at any time I spose, but the main difference is that usually communities are topic focused rather than person-focused, which is one of the main differences between the reddit and twitter models.