I believe kbin allows you to do this, but not Lemmy. Lemmy seems to be very focused on the communities aspect of things.
If both a group-centric UI as well as the ability to follow microblogging users is important to you, consider checking out kbin.social.
There is also always a flurry of people trying out accounts in multiple instances whenever there’s a migration wave, so not only are we seeing people who dipped a toe in only to leave, or go back to Reddit, but we’re seeing the effect of people understanding how the ecosystem works better and settling into a single active account.