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Community Builder, Acymailing5, BreezingForms and Jcomments are additions, I often used with JEM.
For Community Builder are Settings in JEM. It works with the free version too (only registering is needed)
For Acymailing5 there is a plugin which connects with Acymailing5. It works even with the free version. If you need more comfort, you have to pay for.
BreezingForms can be used to add an attending form when the attending person don't want to register into your Joomla site. You can even use the free version. In our forum you can find how to add a button for that.
Jcomments seems not to be continued, there are some problems, I could not find a solution.