How is it native speakers use 'if you want to join me' like this?
I've got some groceries to unpack downstairs if you want to join me.
- Movie, "The Boy"

I'm probably going to grab a quick bite to eat before I get ready for bed if you want to join me.
- Book, "Blooded Crown"

There's a bottle of wine and some mince pies there if you want to join me.
- Book, "With Love at Christmas"

To be honest, the sentences above all sound weird to me.

I think they should be:
- I've got some groceries to unpack downstairs. Do you want to join me?
- I'm probably going to grab a quick bite to eat before I get ready for bed. Do you want to join me?
- There's a bottle of wine and some mince pies there. Do you want to join me?

What do you think?

Question

How is it native speakers use 'if you want to join me' like this?
I've got some groceries to unpack downstairs if you want to join me.
- Movie, "The Boy"

I'm probably going to grab a quick bite to eat before I get ready for bed if you want to join me.
- Book, "Blooded Crown"

There's a bottle of wine and some mince pies there if you want to join me.
- Book, "With Love at Christmas"

To be honest, the sentences above all sound weird to me.

I think they should be:
- I've got some groceries to unpack downstairs. Do you want to join me?
- I'm probably going to grab a quick bite to eat before I get ready for bed. Do you want to join me?
- There's a bottle of wine and some mince pies there. Do you want to join me?

What do you think?

Answer

Okay this might take some explaining.

Using 'if' is more of a cultural question than a grammatical one.

The authors say 'if' if they're close to (in a friendly way) to the person. It's essentially an informal way of asking the question "do you want to join me." It also leaves the question open, just in case the person asking is unsure of the response.

It's also more frequent when spoken, rather than written. In a message to friend you may well begin another sentence with 'Do you,' but as a native speaker, I would always ask 'if'.

eg. 'I'll be on Skype later on tonight if you want to join me.'

Answer

In my dialect of English, I can say this, though the first example is a little weird. I think I'd consider this kind of phrasing a very casual open invitation amongst people you know very well. These phrases with the question, "do you want to join me?" is also correct, but it sounds more polite. I would respond politely with a yes or no, whereas with I have a bottle of wine if you want to join me sounds less like a request or question and more open-ended. The speaker is offering or wanting company, but the recipient is not necessarily obligated to respond directly.  Does this make sense?