Specific synchronization when sharing

I have two machines M1 and M2, which synchronize a folder F via Seafile.
I am sharing with some people (M3) a folder in F, let’s call it F/S.

I am adding some files newf in F/S. Then the people are able to see these new files newf.
I would like to prevent this last statement.

More precisely, concerning the new files newf, I would like two features :

  1. M1 and M2 are synchronizing them, that is both machines will see the new files newf
  2. The other people (M3) shouldn’t see the new files newf

If I add to my seafile-ignore the instruction:
Then point 2 is satisfied but not point 1

Any suggestion ?