Web/mobile: Chat.cc is an easy way to create a chat room for unlimited users. All you need to do is email people and CC "[email protected]" (without quotes). Everyone in that message gets an invitation, and they can sign in with their email address.
Disposable chat rooms are nothing new, and there are already secure options like Chatstep. While Chat.cc also claims to be secure, its best features are the mobile and web compatibility, easy signup and invitation, and the ability to embed rich media. Plus, it is designed well and looks attractive, which is always impetus to keep using a service.
You can tag users in the chat room to draw their attention. Paste links to YouTube, Instagram or any website and they will auto-open in a preview pane. If you leave the chat room, go to Chat.cc and sign in with your email address to see all the rooms you were invited to.