Two different models: open poker rooms vs. clubs
It helps to separate two basic models. Open poker rooms like PokerStars and 888poker are public platforms where every player registers directly on the site, joins the same global pool of tables, and plays under the operator's standard terms. The club model, on which GGPoker's ClubGG app and similar networks are based, is built around private, closed clubs: each club is run separately by a local agent who approves membership, tables open based on community demand, and the atmosphere can feel closer to a known group than an anonymous site.
Both models rely on similar underlying technology (table software, security, GGPass in GGPoker's case), but the joining experience, local pool size, and human support differ substantially.