For information on the room key, see room key.

If you have a studio ID, you can visit the studio with the specified studio ID directly from the ring-menu.
Also, you can visit a studio whose entry is ON even when the studio is not on air. This is useful when you already know the studio you want to visit.

You can copy the studio ID of the studio you are currently in from [Studios] > copy_studio_id from the ring-menu.

You can see the studio IDs of the studios currently on air from the broadcast listing(Only available in Japanese).
Mouseover on one of the streamer icons to see the studio ID for the respective studio.

Ask your friend to check their studio ID following the before mentioned steps and tell you.

On the title screen, enter the studio ID on the Destination room key/studio ID field. After entering your studio, on the ring-menu, select [Studios] > Specified Studio to visit the studio of your interest.