When did you come here? Welcome to join us.

[Silverobama]

My friend Shan joined my chat group a few days ago but I didn't notice that. Tonight, he posted some messages and I saw him. I asked him "Shan, when did you come here?" And he said "A few days ago". I replied "Welcome to join us".

Are my sentences natural?

 

[emsr2d2]

No.

When did you join?
Welcome/Welcome to the group.

 

[teechar]

"Welcome to join us".

If you add a couple of words at the beginning, that can work in a different context. As an exercise for you, can you figure it out?

 

[Silverobama]

If you add a couple of words at the beginning, that can work in a different context. As an exercise for you, can you figure it out?

Let me give it a try: you're welcome to join us. This is perfectly fine when I want someone to join, say, my birthday party.

 

[Skrej]

Deviating from the original question a bit more, but if you're in an actual physical gathering and you just noticed someone, you could ask "When did you get here?"

It suggests they're either a late arrival or you completely overlooked their presence earlier in the event.

 

[teechar]

my birthday party./

Not really. For that, you might use "(and) you're invited (of course)".
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This is the kind of scenario I had in mind:

A- So, what will you and your cousins be doing later tonight?
B- We're having Pizza at Mario's. You're welcome to join us (if you like).

It's a way of inviting someone.