It's easy - just select the desired access level, copy this link and send it via email, instant message, etc.Link: Just a link to your wiki. Post this anywhere and people can ask you for the invite key.
Individual sites will each have their own informative page on Roman Britain.org
A longer list of sites is also published by Athena Reviewhere and here. (Warning - old page. Although the list seems comprehensive, almost all the links are dead.)
Wikipedia has a list of sites with links to further information on almost all of them.