RIBE Scheme Implementation - How to encrypt message
Given that the authors (as any sane person should¹) use multiplicative notation for all of the involved groups, "$\cdot$" simply represents the group operation in the target group $G_2$.

It does not seem to be explicitly specified as far as I can tell from skimming the paper, but this implies that the message space of the encryption scheme is exactly $G_2$, meaning that we always have $M\in G_2$. So there are no "chunks" involved.

If you want to encrypt a message directly with the scheme, this means that you will need to encode it as a $G_2$ element. If your message is too long for that, you will need to use one of the standard domain extension techniques, such as using the RIBE as a KEM in a hybrid encryption scheme.



¹This is a hill I'm willing to die on.