How to increase yahoo group member?

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first you should have a clear idea about what you want the group to be used for. Write that down in the group's description and make sure it is "listed in directory". Then think of what kind of people you think would be interested, and then try to reach those.

Having clear and practical rules (for instance no adhominems, no TOFU) and enforcing those, helps.
Chosing the right settings ( "new members" rather than "anyone can spam") help. Privacy-settings depend on the type of group.

O, remember size isn't everything. A small list with low but interestng traffic often is better than a busy one with a lot of "static".