The game just knows that you two will end up fighting,and then there will be tears,and after that you will both end up being grounded or sat on the norty step.
OR
If you are trying to log on with the same PC,you probably cant do that at the same time-
OR
If you are on your PC and he is on his,but he is logged on as unavaliable(red dot by his name),you will not be able to invite him
OR
If you mean invite him to join Wgt and you both use the same PC and / or email addy,maybe Wgt has a stop in place for that action?
Lizziex