Finally! People have the balls to try some Old Norse! The only way to improve your language is to try, so people can point out where you went wrong. When these mistakes are learned, then they will not be made again... by this principal, surely this could be repeated until your skills in the language are perfect!
Anyway, Auðvarðr... just a few small points.
1) Plural of "vinr" is "vinir".
2) Try not to forget that Heill is an adjective... and that "Heil vinir" could mean that the "Vinir" are "Heil".
In which case, if you were saying "Hale/Healthy friends", vinir is masculine plural, so it would be "Heilir vinir".
But obviously, you're saying "Be healthy, friends" So they must be used as two separate phrases.
In essence, all you need to do is put in a comma. Heil, vinir.
It's the same difference between "Good, thanks" and "Good thanks"... the latter implies that the thanks were good. :P
3) If you're trying to pluralise "Hvernug hefir þú þat?" in Old Norse you have two pronounal plurals...
Dual pronouns "We two" and "You two", or standard plural pronouns "We many (more than two)" etc.
If you're addressing ALL the members on the forum, we can easily assume you want to use the standard pronoun... it's not as if there are only two people you're addressing.
So you choose "Þér" which is the standard 2nd person plural. (Þit is the dual pronoun)
Hvernug hafa þér það?
But the verb "at hafa" is conjugated as...
Ek hefi, Þú hefir, Hann hefir, Vér/Vit hǫfum, Þér/Þit hafið, Þeir hafa
(also, það is Icelandic... the Old Norse is "þat")
So it should be Hvernug hafið þér þat? (if addressing two people... Hvernug hafið þit þat?)
Finally, if you're still addressing the whole group... Tala is the imperative singular form... to order one person to speak. To order many, you use the imperative plural form. Which in this case would be Talið. And in Old Norse, it seems common to include the pronoun for PLURAL imperatives... (I think so at least)... and folk has an accent in Norse... Fólk.
Everything else seems pretty good.
Well done, bróðir. This is also a great way for me to learn. As I make corrections, it reinforces rules in MY head.
So what you should have put was...
"Ek vil prófa þetta! Svá...
Heil, vinir! Hvernug hafið þér þat? Talið þér, fólk!"