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My user should only create events inside of groups, not on the wall... Many users don't understand the difference between a group event and other events. So they create them at the wrong place.
I'm very sorry but this's not possible out of the box and it will require php code modifications which are beyond our support scope. So maybe you could consider hiring a professional php develope for this task, check our developers page here for list of recomended freelance developers >
www.jomsocial.com/jomsocial-developers
You can also propose that idea to our uservoice page here > uservoice.jomsocial.com and if this feature will be popular within our customers, then maybe our devs will consider it as future improvment.