Yes, it is valid. I did a quick search and found "Everyone is welcome along" in newsletter/event announcement contexts, however none of them appeared to be American. The usage may not be regional to the United States, but from what I found in "social media" genre usage (casual register) that included audiences in Australia, New Zealand, Scotland, and a few sites with the ".uk" suffix, it is valid.
I think U.S. English users are more accustomed to the "to come" inserted before the "along" OR they want the "along" to be followed by another phrase like "for the ride", or "to the fair". My guess is, as you have written it above, it feels incomplete to American ears and tongues, but I believe it is an issue of regionality, or of being regionally dialectic, or dialectically regional...or should I should I just say "regional" or "dialect"...? (And no, I'm not talking about Hegel's hood. Please forgive me my linguistic dilletantishness - or is it dillettantism - and my qua linguistic cant.)
Nonetheless, I don't think it has to do with a naturally occurring gap, in which case if it does, it would still be grammatical, or not ungrammatical; that is to say a naturally occurring gap, though not of standardized (or standard) usage, could indicate this is an emerging form that has yet to be analyzed and formally codified.
But, if its formality and codification you want, enjoy the following:
-- From my old and fabulous 1971 Webster's Seventh New Collegiate Dictionary:
3: as a companion or associate
< brought his wife ~ >
So, with this usage (or is it use?) the substitution works:
Everyone is welcome along.
= Everyone is welcome _____ (as a companion or associate).
p.s. I do like the way you chose grammaticality in your question, and that you tagged it as such. I've always understood that term to describe one's own meta-awareness and level of facility with a grammar; referring to one's own fluency as a user of grammar. I didn't realize that this forum defines it as referring to an utterance and its conformity to grammar. Your question, to me, is asking three things; you're wanting to check your own grammaticality, and the grammaticality of the sentence, while you're also asking for proof. Perhaps next time, tag for both grammaticality and grammar, lest your nonconformity to tagging etiquette irk the ire of those who would accuse you of opportunistic pendanticality. = )
- "A speaker's linguistic competence, which is the knowledge that they have of their language, allows them to easily judge whether a sentence is grammatical or ungrammatical based on intuitive introspection. For this reason, such judgements are sometimes called introspective grammaticality judgements." (Grammaticality - Wikipedia, the free encyclopedia)