You're going about this in a slightly wrong way. The permissions on a directory (what you call a folder) only affect if users can list, delete, and create files in that directory. They don't affect what users can do to the files (e.g. whether they can edit them or run them). Only the permissions on the files contained in the directory themselves determine that.
If you chmod the directory 0755, only the owner can create or delete new files in the directory. Users in the group and other users will be able to change into the directory and list its file.
If you want to allow members of the group to append files, you must do things. First, they must have write permissions on them. Second, you must set the file append-only using "chattr +a". You'll need to note two things: (1) I believe only the root user can set a file to be append only (this may be a setting somewhere), and (2) this setting applies to all users, even the file's owner. There's no way that I know of to have a file append-onky for some users and not for others. To do what you want, you might need to allow the file's owner to unset the "append-only" attribute and then re-set it (this could be done using sudo).
So setting the directory to 0755 is a good start. However, you must then set the permission on each file in the directory to 0660 for normal files and 0750 for scripts. Then, for regular files you must use chattr to make them append only.
Note that this assumes that no other ACLs (see "man setfacl" and "man getacl" for details).