If you are an admin, configure your license settings by going to team > licensing > and select whether you want your users to be able to invite other users to your team. You can restrict it to be by invitation by an admin only.
If you have it enabled for users, they will be able to invite them by typing in a college's email on the home page.
As an admin, to invite a user, whether the user(s) have an existing account or not, go to the + user page and chose a method that you want to add the users to your team. Learn more about adding users.
Please note that if the user you are inviting already has an existing account, they will need to accept your invitation until you can manage their settings or see them in your team.
If you have sent an invitation and have yet to hear back, you can remind the users by going to your user management page, selecting "pending invitations", and right-clicking on the user(s) that you want to remind, and select "remind user"