Only Vault Admins can invite fellow Vault Admins.
Step 1: Navigate to Sell side;
Step 2: Navigate to the Actions menu in the Vault Admin row and select Add users;
Step 3: After the warning message you will be able to fill in the email address;
Step 4: Fill in the email addresses and determine the Q&A role in the next step. Be advised to make all Vault Admins a Q&A Admin;
Step 5: Select Invite now to send the invitation.