What should I do if I think two people are working together (colluding) at the table?

A. Every site monitors for this so it is very rare but it is theoretically possible for two people to be working together at the table in some fashion. If you believe this is the case, send the site an email listing:

the limit you're playing
the table number
the account(s) of the people you would like to report

Even if it turns out that they aren't doing anything wrong, with thousands and thousands of tables around dozens and dozens of online poker rooms - just change tables if you think something is funny.