I want a private place where I can talk to specific people.

I’d imagine I want something like:

• By default, nobody can register a new account on my server
• By default, nobody can view or join the rooms on my server
• If a friend has an account on a different matrix server, I can invite them to mine

I probably want some kind of federation with other instances (eg, where my friends might register their accounts), but not some free-for-all. Can someone recommend the right settings? The server is running synapse.