Your team member needs a Nettla account and must be on your club's player list. Then go to Settings → Administrators → Add administrator, find them and choose their permissions. There is no invite-by-email on the Administrators page.
Step 1: Get them onto your player list
Choose one of these:
Send them your Player activation link. Click the link icon in the top bar and copy Player activation link. When they register or sign in from it, they join your player list.
Invite them by email:
Go to Players → Player List.
Click Add Player.
Enter their Email address.
Click Invite.
They must open the email and create their account (or sign in) before they appear on your player list.
Step 2: Make them an administrator
Go to Settings → Administrators.
Click Add administrator.
In Find Nettla user, type their name and click them.
Under Permissions, untick anything they should not have. All permissions are ticked by default.
Click Save.
They get an email saying they've been added as an admin, with a Manage your club button. They log in at operator.nettla.co with the same account they use in the Nettla app.
Good to know
You need the Administrators permission to add administrators.
If you can't find the person in the search, they are not on your player list yet, or they haven't finished creating their account.
You can only invite the same email to your club once every 24 hours. If you try sooner, you see "Recently invited".
Each permission gives both view and edit access. See "How do administrator permissions work?".