You can find the See app in the Slack directory or by tapping this link:
After tapping the Add to Slack button you will be prompted to allow permissions to your Slack workspace.
When you accept you will be displayed a confirmation page.
To use See inside Slack you just type the /see command and you will get a button with a link to your See screen share and video conference.
To invite specific people to a See session you first just create a group with them in and then type the /see command