There are a few ways you can do so at different times during the contest:
1) During the contest, you can use the commenting mechanism provided by the site in each Designer's submission area to comment on specific designs. You can also use the contest comment thread to make general announcements or requests that don't need to be logged as brief updates.
2) During the Finalists and Selection phase, you can use the commenting mechanism provided by the site or use the Private Message feature to contact Designers is you find the commenting mechanism is not getting responses, or if you need to share some more private information. Please do not share emails until after the contest is complete.
3) After the winner is chosen and the files are approved, the contract will populate in the Files Tab in the contest page. Designers and Clients can see each other's emails listed there. This can help with facilitating requests to get extra file types not provided as part of the regular contest package. You can also use the Private Message feature to exchange emails to help with establishing communication after the contest.