Does PostgreSQL allow multiple grantees in GRANT commands?
Yes it does, but that will turn into a maintenance headache down the road.

Read http://en.wikipedia.org/wiki/Role-based_access_control for justification for assigning rights to roles over individual users.

Simply create a role and assign users to it, and then grant privileges to that role.

create role some_group;

grant some_group to mary, bob, sue;

grant select, update on a_table to some_group;