These buttons allow you to do as follows;
Group Video Button allows you to Group slides onto a Video Background so that you can do text over video.
Ungroup Video Button allows you to separate the slides in the grouped video back out so that you can add/remove slides and add, remove, or fix the text.
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Let's group then ungroup our lyric video slide. We will use the group video button to complete a text over video slide.
After you have added the video and the slides with the text on them, select all of them by clicking the top slide, then go down to the last slide that you want in that group, press the shift button, and click on that slide. Now, click the group video button. Choose your transitions if you like.
Afterwards, click OK. And here it is. We will use the ungroup button when we want to delete or add content to individual slides, or if we want to change the background video.
To do this, simply select the group slide you want to make changes on, and click ungroup video. It will look like this. Then you can do your changes and regroup your slide after you're done.
Updated