Friend? Enemy? Whatever -- Nikki Bella has one message for Ronda Rousey ... get your ass in the ring! 

The WWE superstar was finishing up a shopping spree at The Grove in L.A. when we asked about rumors Ronda could be joining Vince McMahon's family biz. 

"I'd love to fight against her ... or with her!"

Bella says she's not sure if Ronda will actually sign a deal -- but she hopes so! 

Just last week, WWE honcho, Triple H, told ESPN he knows Ronda's interested and would love to give her the opportunity.