Allow Invite button to be enabled by default

What does this MR do and why?

• enables the invite modal button by default(unless over free user limit) for clarity see https://gitlab.com/gitlab-org/gitlab/-/issues/368014
• adds form input error if invites aren't added when Invite is clicked.

Screenshots or screen recordings

action	before	after
initial form
submit	can't

new experience

How to set up and validate locally

1. Go to the members page for any group or project you are owner of.
2. Click Invite members button
3. Click Invite on the modal
4. Observe error and functionality.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #368014