is there a way to find out if an element is exist in an array

As part of my project, we have to build a tool similar to phppgadmin, I want to find out if a particular is in what group, I used "select grolist from pg_group where groname = 'webmaster';" it returned an array of user in group "webmaster" I want to find out if a particular user is in that array.

Thanks in advance

-Brian