Yes, you can invite new colleagues at any moment by clicking on the invite button at the top of the web portal
Every free trial comes with 5 free Administrator and Field Technician licenses and 10 Backoffice licenses, as well as 10 subcontractor credits. You will have to subscribe if you want to go beyond those numbers.
When inviting a colleague, her or she will receive an email asking to login and create a password. From then on this colleague will be registered in the system.
Read more about how to invite a colleague.