Only Vault Admins can invite fellow Vault Admins.
Step 1: Navigate to Roles & users or Sell side;
Step 2: Navigate to the Actions menu in the Vault Admin row and select Invite 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.
Step 5: Select Send to send the invitation(s).