The following article will walk you through the steps of adding a Front user who is not currently a guest users in Teams.
Front uses SSO through Microsoft so if a volunteer is not using Teams, you will need to create a guest account ins Azure in order for them to be able to log into Front.
To add a user to Front you'll first need to set them up as a Guest User in Azure.
Go to portal.azure.com
On the homepage, click 3 Horizontal lines.
On homepage menu, click Active Directory
Under manage, click Users
On the Users page you will click New user.
On New user pull down menu, click Invite external user
Type in their email address (and personal message if you want) and click Invite.
They will receive the following email invitation.
On the users page you can search for them and see their Guest account listed
Once they accept the invite and create their Microsoft account they will be able to successful log into Front via SSO.