I'd write something like: People were going backwards and forwards ordering drinks. To and fro doesn't work in this context. 'Up and down' could work though 'People were up and down ordering drinks.' I think that sounds ok.

"To be" is a very static verb, and so "were" doesn't sound exactly right here. "To and fro" and "up and down" clearly indicate movement, so you need a verb which indicates that. "Were" could work, but not automatically.