Sorry, inviting your colleagues (i.e the team feature) is not supported in our Basic Account (free tier). In fact it is also not available in the next tier - i.e the Individual plan. As the name suggests, both these plans are single user plans. That being said, your colleagues can independently sign up for their own Basic or Individual accounts.
Please consider upgrading to a Team plan so that you can invite your colleagues. You can also benefit from lower per user fees.