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I would like to edit the header on the emails that are sent out to people, like the invitations to join. I would like to insert my own text to make it sound more inviting in all the emails that are sent by jomsocial. Where do I go to find these?
The header email is be set for all emails (components/com_community/templates/jomsocial/layouts/email.html.php), you can't change it for some email do you want.
For edit text you should edit this file language/en-GB/en-GB.com_community.ini, and find this code and modify as you want :
COM_COMMUNITY_GROUPS_EMAIL_NEW_MEMBER_JOINED="%1$s joined your group, %2$s \n\nTo view the group, follow the link below:\n\n <a href=_QQQ_{url}_QQQ_>{url}</a>"