User management group form

### Before opening, please confirm:

- [X] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-adminui/issues?q=is%3Aissue+).
- [X] I have read the guide for [submitting bug reports](https://github.com/aws-amplify/amplify-adminui/blob/main/CONTRIBUTING.md#bug-reports).
- [X] I have done my best to include a minimal, self-contained set of instructions for consistently reproducing the issue.
- [X] I have removed any sensitive information from my code snippets and submission.

### App Id

djtfurtwddxxx

### Region

us-east-1

### Environment name

dev

### Figma File Version (if applicable)

_No response_

### Amplify CLI Version

_No response_

### If applicable, what version of Node.js are you using?

_No response_

### What operating system are you using?

Mac

### Browser type?

Chrome

### Describe the bug

Behaviours observed:

1. when adding a user to a group that does not exist and clicking add the button goes into `adding user` status then becomes disabled
closing the form and reopening shows the same status

2. no validation on the form where if a auth has email as signup, you can add nothing(yes nothing) and click the add button.
Same for phone number

3. the validation also misses the email format check, you can add a single character it still allows to submit

4. even if we add valid user and submit. re-opening the add user form does not reset the button.

workaround: refreshing the page resets it

### Expected behavior

validation on user groups

### Reproduction steps

1. create a auth resource
2. go to user management
3. create a group
4. click add user

### Project Identifier

_No response_

### Additional information

_No response_

No source files or tests are named. Reproduce the add-user flow from the listed steps in Chrome, then trace the user-management group form's loading and validation behavior. Done means invalid or empty email and phone submissions are rejected, valid submissions work, and reopening the form resets the button state.

Written by the indexing model from the issue text.