To be noted
Under the free plan, any user can invite new team members on Slite.
Under the paid plans, there are admin features so that admins can control who can and cannot invite.
How to invite new team members
Go to your Organization settings:

Bulk invites
You can copy paste multiple addresses to invite many team members in one go:

Auto join
For all team members under the same email address domain to join your Slite organization without receiving and accepting an invitation. Read more here!
SSO
Under paid plans, you can enable SSO for your team. Read more here!