go ahead and (do something)

People use this phrase in casual conversation to express doing something soon, instead of waiting:

You guys go ahead and eat — I'm not hungry.

Let's go ahead and pick a day and time for our next meeting.

everyone

When you need to speak to a whole group of people, you can address them as "everyone":

Can everyone send me your contact information, and I'll pull together a phone list for the group.

How's everyone doing today?

take a seat

To "take a seat" means to sit down. You usually tell someone to "take a seat" when you have some kind of authority over them. For example, you can tell an employee this:

Hi. Come in. Take a seat.

Let's get started.

"Let's get started" means "Let's start."

Is everyone here? OK, let's get started.

It somehow sounds friendlier and more natural to use the phrase "get started" instead of just "start". People use this phrase a lot at work, at conferences, at meetup groups, and so on.