Users and Roles

HI All…
I recently started a WP blog and want to assign people around the country roles where they can post to the site. I thought this was somewhat easy, by applying different roles in My Users area, Author, Contributor, etc. I tried all of them as tests, but unless I give someone administrator rights, when logging in, they get an error message “You do not have sufficient permissions to update themes for this site.”

Was told to download the S2 Members Plugin…I did…seems a bit confusing.

Any advice would be great.

Thank you,