Before you can use group messaging, you have to enable it for your group. Administrators can give leaders permission to enable it for their group, and then leaders can enable messaging for their group.
Administrators can give leaders permission to enable group messaging in the Group Type Settings. .

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If this is enabled, Leaders can turn on messaging from the Group Settings whenever they are ready.
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If it is left disabled, only Administrators can turn on messaging for each group individually.
Enable group messaging from the Group Settings page.
Note
If leaders have been given permission in the group type, they can enable messaging; otherwise, only Admins have permission to enable it.

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Check to enable messaging for the group.
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Choose whether members and leaders can create new messages or only leaders.