When in doubt, use Välkommen. It is sometimes used for a group of people. The only time you have to use välkomna is when you specify "ni" or "vi." For instance, if you open your front door to a group of people you can say "välkommen," but if you send out an invite adressed to multiple people, like a couple, you should use "ni är välkomna"
Not surprisingly, this Latin script of ours isn't entirely fitted to write Swedish, which has several more vowel sounds than did Latin. Thus, to be able to write Swedish, the vowels å, ä and ö were added. They are letters of their own and not in any way some kind of variants of A and O! You can find some sound guidance here.
I don't know what that page was, but I really recommend these videos by our ex-contributor Blehg: https://www.youtube.com/channel/UCUb87YTKOTgnGcAM4toC-6A/videos
There are three on vowels alone. :)