I tried to use the group tool and select train stations, but nothing happens? Whats most likely the thing I'm doing wrong?

The group tool only selects stations that are either Provider or Requester stations, which is the P or R in the Stations tab. Thats really the only thing I can think of right now.

When I click Save after selecting five provider stations, it just says "Error saving group".

A group consists of one provider and x amount of Requester stations, if you don't have any requester stations selected, you can't save the group. (you can save multiple provider stations with the same requester stations at the same time)