  Relative Rota-Baxter groups are generalizations of Rota-Baxter groups and
share a close connection with skew left braces. These structures are well-known
for offering bijective non-degenerate set-theoretical solutions to the
Yang-Baxter equation. This paper builds upon the recently introduced extension
theory and low-dimensional cohomology of relative Rota-Baxter groups. We prove
an analogue of the Hochschild-Serre exact sequence for central extensions of
relative Rota-Baxter groups. We introduce the Schur multiplier
$M_{RRB}(\mathcal{A})$ of a relative Rota-Baxter group $\mathcal{A}
=(A,B,\beta,T)$, and prove that the exponent of $M_{RRB}(\mathcal{A})$ divides
$|A||B|$ when $\mathcal{A}$ is finite. We define weak isoclinism of relative
Rota-Baxter groups, introduce their Schur covers, and prove that any two Schur
covers of a finite bijective relative Rota-Baxter group are weakly isoclinic.
The results align with recent results of Letourmy and Vendramin for skew left
braces.
