Groups on elgg 1.5

It seems that groups on elgg 1.5 have only two options for permissions: Open: everybody can join to read and write content Closed: Everybody must request membership to read/wirte content It isn't more limited than in elgg 1.2? I need to make groups that everybody can read contents but only few user could wirte in... How I can make this on elgg 1.5? I need to write a new plugin?