To invite a friend to the game:
Open your profile (tap the icon in the top right corner) → Go to the Friends section (icon with two zombies) → Open the Add Friends tab → Tap Invite → Copy the link and send it to your friend.

To add a friend who is already playing:
Ask your friend for their Player ID → Open your profile → Go to the Friends section → Add Friends → Tap Add Friend → Enter your friend’s Player ID.

Alternatively, you can copy your Player ID from there and send it to your friend so they can add you back.

To add random players:
Open your profile → Friends → Add Friends → You will see a list of recommended players → Tap Request to send a friend request.