Mr Arindam,
no..having email on any specific email-service provider is not required at all. try looking for settings.
he has sent an email request to u instead, from his yahoo account. U just need to approve his access.
In group email systems, there is never such a compulsion, because the system will lose its purpose. Person having email on any service-provider..yahoo, gmail, rediff, sify...whatever can join group on any of the service providers, although his controls on group may be very very slightly reduced.
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Manish.