nostr:nprofile1qqs2wguqtndxwfg3j8y8sm6d5k8hjlnfwavzxqf4fw5wjx7tqdpde8qpz9mhxue69uhkummnw3ezuamfdejj7qgkwaehxw309aek2arg9ehx7um5wgcjucm0d5hszxnhwden5te0wp6hyctkd9jxztnwdaehgu3wd3skuep0tc03pf had the basis of a client centered around this, in the works, at one point. Maybe someone could pick up & expand on that work?

https://chachi.chat

https://flotilla.social and

https://0xchat.com/ all support private groups. Those might be worth looking into also.