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.