Beware of organizations that promise 501(c)(3) status in a very short time, such as 7-30 days, or for a very low price. It is possible that these are organizations that want to pull you into their group exemption. In this case you will not have your own 501(c)(3), and this participation may not even be legitimate.
As you will see from the link to the IRS pages on the subject, group exemptions are for related organizations that are very similar to each other in their structure, purposes and activities. The central organization generally supervises or controls its many chapters, called subordinate organizations. An example of this would probably be the Rotary Club. To qualify for a group exemption, the central organization and its subordinates must have a defined relationship. Subordinates must be affiliated with the central organization and subject to its supervision or control.
Also, there is no way to check to see if you really are included under the group exemption except by asking the central organization. The IRS does not make available the list of those subordinates included in the group exemption letter.