Its actually simple. Tell them, its like Email. You have an email account at gmail, but can perfectly fine have email conversation with someone on outllook. Lemmy instance = the same as a web email interface of any email provider. Most people will get their head around that.

I don't think such an aggregator is required. Interoperability is smooth enough that you don't have to think about different instances most of the time. I've only really noticed two points that would be confusing:

• the sign up process
• the "local"/"all" distinction

So I think what we really need to do to make this platform intuitive to people that aren't already familiar with it is:

• Somehow streamline signing up. The process from googling Lemmy to having an account on an instance should not be confusing or intimidating.
• Filter by "all" by default. The default should cater to the users which are less likely to figure it out themselves. If you don't understand what instances are and what "local" vs "all" means, then you are probably here for the "all" experience. If you understand and really want "local" you are probably fine having to set it yourself.