“Meanwhile, can you guys decide on the theme?”

You're working on a school project with a group of other students. You are going out to pick up some supplies. You want the other group members to work on something while you are gone. After you offer to go to buy the supplies, you say:

Meanwhile, can you guys decide on the theme?

Meanwhile

You use the word "meanwhile" to describe something that's happening (or will happen) at the same time as something else. This word is often used when telling stories. For example, in this scene from The Last Dickens: a Novel, two actions are going on at the same time:

As Mason made his way around the thick shrubbery he jumped out at Turner's signal and displayed his sword at the surprised Narain, who put up two trembling hands and lay flat on the ground. Meanwhile the other thief had pushed Turner down and dashed into the dense trees.

In the example at top, the speaker wants the group members to make a decision at the same time that he is at the store shopping for supplies.

you guys

People use "you guys" when talking to a group of people that may include men and women. This is still considered casual English, but it's very common for younger people and people from the West Coast of the U.S. to use this phrase.

decide on (something)

To "decide on" something means to make a final choice about something. For example:

It took several months for us just to decide on a new name.

theme

A "theme" is an idea that unites all the parts of a project, a book, a film, or a party.