I think you have two options:
Insert one or more judiciously chosen \allowbreak directives inside the inline math equation to allow line breaks after one of the commas, or
Convert the inline equation to a displayed equation, as (i) the equation may be sufficiently important to merit this treatment and (ii) you're dealing with fairly narrow columns.

\documentclass{IEEEtran}
\usepackage{amsfonts} % for "\mathbb" macro
\begin{document}
A bilinear pairing defined to be
$\mathcal{G}=(p, \mathbb{G}_a, \mathbb{G}_b, \mathbb{G}_T, e, \allowbreak P_a, \allowbreak P_b)$,
where we choose $\mathbb{G}_a=\langle P_a\rangle$, \dots
\begin{center} --------- or --------- \end{center} % just for visual interest
A bilinear pairing defined to be
\[
\mathcal{G}=(p, \mathbb{G}_a, \mathbb{G}_b, \mathbb{G}_T, e, P_a, P_b)\,,
\]
where we choose $\mathbb{G}_a=\langle P_a\rangle$, \dots
\end{document}
Addendum, inspired by a comment by Manuel. While it may be a bit overkill for the immediate example at hand, it may be useful to have a function or macro which enables line breaks after commas when in math mode. If switching to LuaLaTeX is an option for you, the following code should be of interest to you. It sets up two TeX macros named \breakatcommas and \nobreakatcommas which enable and suppress, respectively, line breaking at commas. Issue \breakatcommas before the line(s) that should permit this kind of line breaking, and issue \nobreakcommas after the line(s) in question.

% !TEX TS-program = lualatex
\documentclass{IEEEtran}
\usepackage{amsfonts} % for '\mathbb' macro
\usepackage{luacode,luatexbase}
\begin{luacode}
function breakatcommas ( line )
line = string.gsub ( line, ",", ",\\allowbreak" )
return line
end
\end{luacode}
\newcommand\breakatcommas{\directlua{luatexbase.add_to_callback
( "process_input_buffer", breakatcommas, "breakatcommas") }}
\newcommand\nobreakatcommas{\directlua{luatexbase.remove_from_callback
( "process_input_buffer", "breakatcommas") }}
\begin{document}
A bilinear pairing defined to be
\breakatcommas
$\mathcal{G}=(p, \mathbb{G}_a, \mathbb{G}_b, \mathbb{G}_T, e, P_a, P_b)$
\nobreakatcommas
where we choose $\mathbb{G}_a=\langle P_a \rangle$, \dots
\end{document}
\allowbreakin the list expression, and/or have a look at http://tex.stackexchange.com/q/1959/15925 – Andrew Swann Apr 12 '15 at 09:10[10pt]after{IEEEtran}? – Mico Apr 12 '15 at 09:42