...
The external user needs a Microsoft account in order to join the SharePoint site. Microsoft accounts can be created and used for free until the user needs more cloud storage. Your data as a UConn user does not count against an external user’s storage quota.
Adding external users
External users cannot request access to your SharePoint site; they must be added by an Owner of the site.
Navigate to outlook.com and sign in with your UConn email and NetID password.
In the left-hand menu, click on the Groups icon. This icon has three people shown.
Find and click on your SharePoint site name in the list of Groups.
Click on Add Members.
Type out the email addresses of those you would like to add to your SharePoint site.
Click Add when you are done.
You may close this window now.
...