Hi,
You can use just one public IP to NAT (PAT) all 70 users. You can save the other seven addresses from the pool assigned to you for future use. If there are devices, like servers, that need static one-to-one NAT you can use an address from that pool.
You may need a configuration like this:
int f0/0
description 'local lan'
ip nat inside
int s0/0.1
description 'office_1'
ip nat inside
int s0/0.2
description 'office_2'
ip nat inside
int s0/0.3
description 'office_3'
ip nat inside
int s0/0.4
descripton 'interface to ISP'
ip nat outside
ip nat pool cisco xx.xxx.80.170 xx.xxx.80.170 netmask 255.255.255.248
ip nat inside source list 150 pool cisco
access-list 150 permit tcp any any eq www
Hope that helps!
Regards,
Sundar