I hope you are doing well. You will only be allowed to invited members into your org if you have purchased additional seats to your organization.
Below the section of where you sent the screenshot it will say:
1 assigned--- 0 (or how many seats are left) available
If you would like to share content with someone who do not have an account, another option would be to share the maps publicly. If they do have an account in another organization you could create a group and share content with them through there.
Please let me know if you have any questions.
Thanks that was helpful, I think the group users can't use the shared app from the same link, ArcGIS Online generate a link for each user otherwise i need to pay for seats in my organization.
if u have some knowledge about time layer i will be so thankful if u helped me here https://community.esri.com/thread/250993-continuous-growing-symbols-in-time-aware-app