After sharing a meeting summary with another colleague, your colleague will have access as an Editor to collaborate on those shared notes.
How to edit a shared meeting summary
Share a meeting summary (see how to here)
You will see your colleague is defaulted as an Editor of the meeting notes
Colleague will receive an email notifying them they have been shared on notes
Notes can be added anywhere within the meeting notes/summary by those with Editor access
Edits will be added as any other note would from your end. Will not be noted that for example, Miguel, made track changes.