Group calls are especially taxing on your internet speed, so a high-speed connection is recommended. If you have a slow internet connection and access to your router, use an ethernet cable to plug your computer directly into the router's ethernet slot for consistent internet.   This will open the pertinent conversation, to which you can add people as you please. You can also click the "plus" sign in the toolbar above the "Contacts" and "Recent" section. This will create a new group. This is located in the upper right-hand corner of your current conversation. Clicking it brings up a menu that allows you to add members to the group. You can also search for specific people by typing in their names. If you are currently in a conversation with someone, adding them to a larger group will bring the rest of the list's contacts to your current conversation. Skype can support up to 25 people (including you) in a voice conversation. Only 10 people can actively appear in a video call. Skype will begin dialing all of your group members. You have successfully completed a conference call on Skype!

TL;DR in Indonesian:
Pastikan komputer Anda terhubung ke internet. Bukalah Skype. Masuklah ke Skype dengan nama pengguna dan kata sandi Anda. Klik salah satu percakapan atau kontak untuk membuka jendela percakapan. Klik ikon orang dengan tanda "+" di sebelahnya. Klik kontak dalam daftar untuk memasukkan kontak tersebut ke grup. Tambahkan kontak lain yang Anda inginkan. Klik tombol Call atau Video Call untuk memulai panggilan konferensi. Untuk mengakhiri panggilan, klik tombol telepon berwarna merah.