if someone signs up for the one site then they are automatically signed up for another site like gmail.

Hello
Actually I m creating two web sites. in which i have to develope similar functionality like google,gmail or any other site of google
if someone signs up for the one site then they are automatically signed up for another site. but I am not getting how to implement it.
Both site are at same domain.

Please help me to solve this issue.


Thanks in advance
Omkar

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