You can associate an activity with multiple contacts by adding a Contacts field to your Activity Type.
Activities can only be logged on one primary Contact or Company. Contacts added to your Contacts field are considered secondary.
Some things to note:
Logging an Activity will only update the custom fields on the Primary Contact or associated Company of the Primary Contact, so as to not add non-applicable information to additional records
When sorting or filtering by logged Activities, only the Primary Contact will be included in the sort
Automations affected by logging an Activity will run on the Primary Contact only
Any Contact included in the Contacts field will have the logged Activity on their timeline and will be linked from the Activity summary
For example, let's say you have a meeting with a consultant on a client's behalf. The Activity fields ask for information on the client, but you want to make sure you track who you spoke to.
In this case, you would log the Activity on the client as the Primary Contact, so that all the information trickles into their contact record. The consultant would be listed in the Contacts field for transparency, clarity, and organization.
Associating an activity with multiple contacts is a great way to execute account-based marketing or sales.