Can it be hat you are trying to add the same user, that you have tried to delete before?
deleting users is not supported in dotlrn. One can ban etc. users, but deleting is quite hard due to the traces in the system (e.g. do you want to delete the forums entries that the user wrote? and the replies to it, etc.)
it would help if you explain step by step what leads to the problem (and submit a bug report to the tracker).
1.- login into dotlrn with a user with permission of superadministrator.
2.- I clicked on "Join/drop a class or community group"
3.- I click on join button of the community in that I want to join.
4.- Appears the error