Plugin to see viewers

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Please tell me what Plugin is used to see from the home page the number of poeple viewing a perticular forum.

For instance, you can see how they show that on the forums of vBulletin:

vB3.5 Plugins (26 Viewing)

vB3.5 Extensions (27 Viewing)

Thanks

[DementedMindz]

wrong forum...

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Oops!!! I thought it was about Plugins :(

[Zachery]

Default vB feature.

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How do I use it?

Anyone knows about that please? :)

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Anybody please?

[Zachery]

AdminCP > vBulletin Options > Forums Home Page Options > Display Logged in Users? > Yes