Teams now supports memberships of up to 5,000 users. This is great for large tenants, but probably isn't too interesting for most of Office 365. If you're in the situation where you might need to operate very large teams, you might need Microsoft to make some changes to the client, write some tools, and impose some basic etiquette on Teams users.
I know what the problem is. If you're a guest user, you don't see a search box for the membership list. It's tied to the idea that guest users shouldn't be able to browse the directory. In any case, I've adjusted the text. Thanks for pointing this out to me. I guess I should always check stuff by logging on as a tenant user and then as a guest!