You can play in those mode by
1) From the lobby screen, click on 'Search Player'.
- If your steam friend is online, click 'Invite'.
- If not, go to step number 2.
2) Type down the nickname of the player you wish to play with.
3) Place the mouse over the nickname and click 'Invite'.
4) If the invitation is accepted, a Duo or Squad is created.
※ If the player is not online, the 'Invite' button will be grayed out.