There is no need to create separate Teams when you have an existing SharePoint site or an Outlook Group. Outlook Groups and modern SharePoint sites all have an Office 365 group. The members of the group are the members of the Outlook group, or SharePoint site.
The process begins the same as if you are creating a new Team. You must start in the Teams tab of the Teams app.
1. Click the Join or create a team button at the bottom of the Teams list.
2. Select the Create a team option from the Join or create a team list of options.
3. Select the team type that best fits your scenario. A description is included below the team type name.
4. At the bottom of the Create your team window, you may see an option called Create a team using a group set up by you or University of Wisconsin-Stout. Select this option to see a list of eligible Sites/Groups.
IMPORTANT: Only users who own either an Outlook group, or own a modern SharePoint site will see the option noted in step 4. If this option does not appear, for you it means either...
a. You are not the owner of any Outlook groups or modern SharePoint sites.
i. Contact the owner of the group/site and request that the owner follow this process to convert it to a Team.
ii. If you do not know who the owner is, enter a Help Desk ticket and the Help Desk will tell you the name of the site owner(s).
b. Any groups/sites you own have already been converted to Teams.
i. If you are in the group, you should see the Team in the Teams tab. You may need to expand your hidden teams to see this team.
c. The SharePoint site you own is too old to be converted to a Team (this could only be true if the site was before May 2016, or if it was created by an admin using an old style template.)
5. Select the SharePoint site or Outlook group by name from the list of options provided, then click the Choose group button. If you do not see an option, refer to the list above for reasons why your it does not appear.