Whatever is required to ensure that the representative is able to carry out their lawful duties: in this case entering, without warning or oversight, ICE facilities.
That could be Congressional ID slid across the counter, an asp to the knee (or whatever), or the sort of force
@Good2Golf suggests with whatever sort of open-ended ROE you might want.
Re: employees of Congress, the access option seems pretty clear if it's someone told off to poke around and report back (24 hours notice). Still curious what the original intent was/what comparable practice might be as far as a Congressperson showing up with their entourage. Does Senator Bloggins have to leave their aides at the gate? Can't really see that being in the original intent.