When using Kast, you need to have kasting permissions in a party in order to stream video & sound and to control Kast TV in that party.
When you join a party and don't have access yet, the buttons will look like this on web:
On mobile, the icons are on the right side of the screen and look like this:
A good way to start is to ask for streaming access in the party chat. The group owner and moderators have the power to give you streaming rights.
Send a polite note in chat, and tell them you have some awesome things you want to share (like videos of dogs falling into swimming pools). Like this:
Once the owner or moderator has given you access, the buttons will no longer be greyed out, and you will be able to share your screen and audio with everyone else. The buttons will look like this:
Now, just click the colorful button on the far left to start Kast TV, the screen icon to share your screen, the camera icon to share your wevcam, or the microphone icon to unmute yourself and broadcast your audio.