I translate from Russian using a translator:
The game already has a Harem of 2 - 7 girls (or guys) + MC, there is already a sex scene - or is that not what you mean?
To do this, you need to Dominate 2 - 7 girls (or guys) and one of them will have a choice in the "Invite on a date" option "Hang out with the subs" - further it is intuitively clear
(haven't tried with Open or Polyamorous relationships)