Google Fonts?

What do you mean by Google fonts? Proboards doesn't necessarily support all fonts that exist; Proboards will support any major font face (I have to find the list; I will post the link when I do find it) and it will show any font that is available on your computer. However, for the latter, it will become default to everyone else if they do not have the font on their computer.

Hope this somewhat helps!

EDIT: Okay, here are some of the links I found. Just note that anything that looks like Times New Roman means that the font probably isn't supported by Proboards.
Luna's font guide
Guide that Saph found
I would be wary of both of these guides as they are a bit outdated (meaning they were created while Proboards was still v4; we are in v5). However, with at least Luna's guide, you can see what made it through to v5.

Okay, so are you asking about the tag? I'm not really sure what you mean by the question. Or better yet, what have you been doing and how is it not working?

use this link

then use this code

[*googlefont="font name:number (usually 400)"]

without the asterisk.

<3

The code I provided only sort of 'downloads' the font into your post. You still have to use the usual [*font face="font name"] code.

[googlefont="Josefin Slab:400"]this is an example

[*googlefont="Josefin Slab:400"][font face="Josefin Slab" color="white"]this is an example[/font]