The new clients join existing shareholders BAA, British Land, Mace and Tishman Speyer.
MEPC, which has a development programme worth £1.2bn, will use Asite's online collaboration, knowledge-sharing and procurement tool on several projects.
Land Securities development chief executive Mike Griffiths said the portal would not only improve his business but give investors greater confidence in the property sector.
Speaking at the conference, Asite chairman and Rethinking Construction author Sir John Egan said the portal was building on the foundations laid by the government-sponsored Movement for Innovation in enhancing the efficiency of the construction industry.
Asite was launched last October.