Oops my bad, turns out I was using755 not 775. Now my problem is only with folders. If one user creates a folder, it ingores the force group parameter and just creates it as that group. This is my config:
[auclair]
path = /data/samba/shared/auclair/
comment = Auclair
browseable = yes
writable = yes
valid users=administrator, auclair
forcegroup smbusers
create mode 770
create mask 770
directory mode 770
[shared]
path = /data/samba/shared/
comment = main share
browseable = yes
writable = yes
guest ok = no
valid users=administrator,ryan, p2puser
force group smbusers
write mode = 775
directory mode = 775
If auclair user writes to auclair share, I log is as ryan into shared and cant delete any files inside folders made by auclair. But just a file added to an existing folder will delete fine.