That's an excellent question.
The answer is that, yes, they can claim the Fifth if
and when they are called before a federal grand jury (and I will bet you
that there is at least one federal grand jury already looking into this
. . . and there may be more, and may be state grand juries, too, before
it's all over).
To claim the Fifth Amendment privilege, you have to
(a) be compelled (b) to give testimony (c) that incriminates you, i.e.,
that provides evidence which can be used to convict you of a crime. A
subpoena--whether a Congressional subpoena or a grand jury
subpoena--qualifies as compulsion, as being compelled. Both before
Congress Lay and the others would be asked to speak, to "testify," so
that element is met, and we can assume, I think, that what they say
would to some degree at least incriminate them.
As I said, I am sure grand jury proceedings are
already going on, in secret, which grand jury proceedings always are.
Congress, which does not have the power to indict Lay or the others,
i.e., to charge them with crimes, is simply making political hay out of
the opportunity to denounce what they did . . . which is probably not a
bad thing. (Especially since Congress does have the power to take steps
to try to ensure that this sort of thing does not happen again.)