What version of Elgg are you using? What hosting are you using?
In any case, here are some interesting threads about this error:
http://community.elgg.org/mod/groups/topicposts.php?topic=598056&group_guid=179063
http://community.elgg.org/mod/groups/topicposts.php?topic=515637&group_guid=179063
And a blog post on how to solve it:
http://evanwinslow.com/2010/11/elgg-troubleshooting-allowed-memory-size-of-67108864-bytes-exhausted/
Rodolfo Hernandez
Arvixe/Elgg Community Liaison
info@elgg.org
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