Gfilui

I have a user user1 that belongs to group1 as primary group, but I need this user to have read/write access to a folder that belongs to group2.

I tried adding a secondary group to this user so it now belongs to group1 as primary group and group2 as secondary group but I still don't have access to the folder.

I used this command to add the primary group usermod -g group1 user1 and this one to add the secondary group usermod -a -G group2 user1

This are my folders permissions:

drwxr-xr-x 8 owner group1 4096 jun 5 10:26 ./
drwxr-xr-x 6 owner owner 4096 jun 5 10:03 ../
drwxrwx--- 14 owner group1 4096 jun 5 11:12 folder1
drwxrwx--- 5 owner group1 4096 may 24 11:32 folder2
drwxrwx--- 13 owner group2 4096 jun 5 10:29 folder3
drwxrwx--- 3 owner group2 4096 jun 5 10:26 folder4
drwxrwx--- 8 owner group2 4096 jun 5 10:25 folder5
drwxrwx--- 6 owner group1 4096 jun 2 09:19 folder6

Can you help me solve this problem please?

If I understand correctly (but it's confusing) I think you've to approach this the other way around. If I understand you correctly you tried to add a group to an user.
What would work is adding the user to a group.
If you want user1 to have access to folders/ files of group 2, do:

sudo adduser user1 group2

Thanks to KamilMaciorowski for the answer I need to modify the smb.conf file and add the new group in order to be able to have access.

I modify it this way:

[PEI]
 browsable = yes
 path = smb/dir
 guest ok = no
 guest only = no
 create mask = 0770
 directory mask = 0770
 write list = @group1, @group2
 read list =
 valid users = @group1, @group2
 read only = no