You can add an attachment while writing an email from the bottom of the editor by clicking the paper clip icon and selecting Browse this computer

 

You can also share files from OneDrive. This will allow your recipients to collaborate on it in real time.

• Click the paper clip icon from new email or calendar event and select OneDrive.

 

• Select the file you want to attach and click Share link. Select Share link from the drop down menu.

 

• If you want, you can change if the recipient can just view or also edit the file by right clicking the attachment and selecting Recipients can view

 

• Under title More settings you can select the option you want, save the change by clicking Apply.

 

More information about adding attachments: Attach files in Outlook on the web - Office support portal