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When you are in the US and you are seeking to adjust your status to permanent resident you are not following to join. You are considered a derivative beneficiary of her approved I-140.
You do not need to file an I-864 as you are not seeking to have her petition approval sent to a consulate abroad.
As long as you maintain your H1B status you can file to adjust status based on her approved I-140 and/or I-485 as long as you and she were married at the time her adjustment of status was approved.
You would file the I-485/765/131 with the Chicago lockbox PO address.
You would send a cover letter attaching a copy of her I-140 I-797 approval notice and her fee receipt for her I-485. If her 485 has been approved then you include a copy of that approval notice.
USCIS has recently established a policy to now interview all employment based adjustments. You can file your I-485 anytime before or after her 485 approval.
It gets confusing when you use the term "follow to join" because you are not out of the country and do not intend to be out of the US to do consular processing. You are, however, her derivative beneficiary spouse because you were in a valid legal marriage at the time she had her adjustment approved.
There is not time requirement for you to file your 485 as long as you maintain your H1B status.
Since you are not seeking to have a consulate notified so you can do immigrant visa processing, no I-824 needs to be filed.
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