Perez is the third DNC chairman since this past summer with the resignation of former chair Debbie Wasserman Schultz after revelations from the party's hacked email accounts, NBC News noted.
Now, Perez faces the daunting task of rebuilding the Democratic party - and unifying the Democratic resistance to Trump.
Since the November election, the DNC has been a forceful critic of Republican President Donald Trump.
The new hires to replace outgoing staff are being guided by a Transition Advisory Committee consisting of about 30 members.
She replaced former Chair Rep. The party moved to the left with Sanders' candidacy and the question was how much clout the liberal left will have once Perez gets done.
In this a year ago, the DNC has gone through three party leaders.
Perez has spent his first weeks on the job in "active listening mode", hearing from Democrats in Washington and in small group meetings across the country before making any big moves.
"We're repairing a plane at 20,000 feet", he told the network, suggesting his view of the magnitude of the task in front of him.
Another source with deep knowledge of the committee's operations confirmed that every employee had to turn them in.
Progressives criticized the committee's initial makeup, leading the DNC add several more members from their ranks.
Perez was selected as the chairman in February.
Sanders' remarks echoed what Perez wrote when he announced his candidacy for Democratic chair in a Mic exclusive.
Schumer, pointing to Ellison and Sanders, told Perez, "If he's happy, and if he's happy, then I'm happy", according to two sources.
Asking for the resignation letters makes the process of deciding who to keep, regardless of how many are eventually let go, much easier. NBC said Ellison's political director has been helping on staffing decisions in key departments.