Those are good services, but I think it is better to stick with an app some people might already use and convenient for them. Whatsapp can be convenient because most people have, but inconvenient because of privacy concerns. I think that's why Discord is a good option, because there will be people in the "gaming" crowd already using it. It is also fairly private, doesn't require phone number and only your nickname is visible to others.
You can have channels, a general channel for talk, a channel for clanners where people looking for clanners will be present and they can play once there's enough people. You can get instant notification by setting channel settings accordingly, but people can also use mentions to make sure.
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