This option is only available to Users with permission to share. You can only share Opportunities with users in the same organization or company as you. Contact Support, or your Consultant if you are unable to share Opportunities. If you'd like to learn about assigning Opportunities
If you are a Channel Manager or Admin of this Install, you can instead choose to change the Ownership of a Opportunity to another User to assign the Opportunity to them so they can start work on it. Navigate to the Opportunities section of your portal under Manage

Note: this may be under a different part of your navigation bar depending on your settings and customization. Contact your Support team or Consultant for further assistance if you can’t find the selection
Select the Opportunity Record you'd like to share
Select the Associated Contact that you wish to share.

Click on "Views" then "Sharing".
Navigate to the "Not Added" or "All" user tabs, then use the add/remove toggles to share the opportunity contact to the relevant users.