When I try to share the PowerApp for the list form I create. I cannot add any users.
Is this a Form, or an App?
And I am not able to open the Share Tab if this is form.
If it is a form, then Sharing option of the SharePoint custom form is managed through the Sharepoint side.
You need to use the Copy Link to share this form.
Please check the FAQ part under the following page:
Q: How can I easily share my form with others?
A: Open the form, select Copy link, and then send the link to anyone you want to use the form.
For the App created, If you are original creator, you should be able to share it:
Share an App in PowerApps
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