this is the xp section but it should work the same for win7

once you connected there will be a network adapter for the dialup
connection in the network connections window

in fact i think the dialup connection icon is there even if your not
connected (been a while) so you right click on that one and proceed
to share the internet the same as if it would be a lan or wifi
connection

i know it works in xp because i have done it many many times (back in
the days)

please note that this might be not as easy if your using AOL, it has to
be a real dialup connection that gets a real IP address

in my case we went to roadtrips and had laptops, at gas stations/motels
they sometimes have phone lines you can dial 800 and local numbers free,
so i dialed up (i used MSN and earthlink WITH OUT they software) and
shared the connection either over lan cable or wifi so we can get all
online with 1 phone line... it worked like a charm every time