How to invite new seats members?
Navigate to Members.
Click Invite link.
Enter email of team member.
Send invite.
How to deal with errors when inviting team members?
"Account already exists" - It means your team member already has an account in Heepsy. Emails that own an account can not be invited as team members of another account.
Ask your team member to delete his existing account.
Invite again your team member following the steps above.