Once you've invited team members to your account (via Settings → Users), you can assign them to transactions.
To assign a transaction
Open the transaction
Look at the transaction header (next to the % completion bar)
Click to add the team member as a collaborator
What happens after assignment
That team member will:
See the transaction in their dashboard
Be able to view and work on all transaction details
Receive notifications for that transaction
Note
Users with the "User" role can only see transactions they're assigned to. Admins and Super Admins can see all transactions but still need to be added as collaborators to interact with them.