Tree versus group

Hi all, i have a stupid question but can you check this ?

When do we need to use LDAP groups versus Tree ?

I mean : I have to built a directory service.

Should I use a tree like :

dc=fr |_dc=enterprise | |_ou=unit_1 | |_ cn=guy_1 | |_ cn=guy_2 | |_ cn=guy_3 | |_ou=unit_2 |_cn=guy_1

Or should I use group like :

dc=fr |_dc=enterprise | | | |_ou=unit_1 | | member : dn:cn=guy_1,ou=people,dc=enterprise,dc=fr | | member : dn:cn=guy_2,ou=people,dc=enterprise,dc=fr | | member : dn:cn=guy_3,ou=people,dc=enterprise,dc=fr | | | |_ou=unit_2 | member : dn:cn=guy_1,ou=people,dc=enterprise,dc=fr | |_dc=people |_ cn: guy_1 |_ cn: guy_2 |_ cn: guy_3

Thanks a lot guys.