Hello @Louise Seddon
You seem to be taking the wrong approach to adding a user.
Can you confirm that the groups in the screenshot have been assigned to related projects as such https://support.atlassian.com/jira-software-cloud/docs/add-people-to-team-managed-projects/?
Either the group must have permissions set in the admin section, or the group must be added to the project via the "people" in the project settings.
Sorry if I misunderstood, I'm trying to work with the limited information provided.
It seems that you do not have administrator rights for that project, adding users is done by going to the project. See attachment for how I add users to a project.
Since I am not sure what procedure you have chosen (assuming this is how it will work), please specify your procedure for adding a user to a project.