Instead of inviting your Sub-Contractor as a Connection, they were set up as a Staff Member in Tradify.
- Head to Settings > Staff Members and make this Staff Member in active (open the staff member and select Deactivate this Staff Member)
- Contact Tradify Support (firstname.lastname@example.org) and ask them to update the username to free up their email address as a username
- Head to Connections and select Add a Connection
Find more information on adding and using connections in this help article