User groups are XenForo's primary method of assigning roles to the users of your forum. This allows you to assign permissions, titles and other customizations to users.
This and other similar questions keep coming up so to flesh the response out a bit ...
Set the Start conversations permission for the Registered user group to No
Create a new user group - let's call it Established members
Set the Start conversations permission for the Established members user group to Yes
Create a User group promotion to promote members to the Established members user group using the User has been registered for at least X days criterion and set it to 7
The same approach can be used for other permissions and criteria.
Always remove the permission from the Registered user group and add it to a new user group, and combine it with a promotion.