It seems like you don't want user id as much as you want searchable user aliases (or persona ids). Basically, a private id (linking your clients), where there are multiple public persona ids for "searching" and adding people to conversations.
Users in conversations are linked by (private id and the persona id at creation), where messages get sent between the clients.
Meanwhile, people (or rather private ids) get added to conversations by using the publicly searchable personas (i.e. any globally unique string). Then for the life of that conversation, the persona is sticky. You could even add multiple personas from the same user to the same conversation if that is necessary. For some the persona id could be phone numbers, full names, online aliases, emails, etc.
People can then hand out different personas depending on the context.
Users in conversations are linked by (private id and the persona id at creation), where messages get sent between the clients.
Meanwhile, people (or rather private ids) get added to conversations by using the publicly searchable personas (i.e. any globally unique string). Then for the life of that conversation, the persona is sticky. You could even add multiple personas from the same user to the same conversation if that is necessary. For some the persona id could be phone numbers, full names, online aliases, emails, etc.
People can then hand out different personas depending on the context.