Yes, you are supposed to tip guides on a trip. Especially the bike tour leaders. It's an incredible amount of work.
This year, the tour we took was with a small, Spanish based company, with owners who hail from the UK. They were fabulous. There were only 2 other Americans on the trip; the rest from Australia, Denmark, Canada. We did not tip the owner/leader, but we did tip the other leader, who was an employee. He was a poor, semi-pro cyclist who worked in the asparagus fields to make money. I know he appreciated it. If we hadn't brought it up, the others on the tour would not have tipped him. After we explained the rationale, they all were happy to do so.
The reason we tip in the US is that service people often get paid minimum wage (or less) and depend on tips to make a living wage, as opposed to EU countries, where waiters, etc make a salary.