You could send to the player a window with online players (windows can show much more people), I've made it once but my players didn't like, because they typed !online and a window appeared on their screen, so they got stuck and couldn't move, and if they were hunting, they could die (because the window does not disappear pressing ENTER).
Another way to do this, is separating messages. For each 10 players, you send one message, so when a player types !online, he/she would get this structure:
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18:04 Players Online: a, b, c, d, e, f, g, h, i, j
18:04 k, l, m, n, o, p, q, r, s, t
18:04 u, v, x, w, y, z
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Yours,
Rafael Hamdan;