The Activity Member component will allow you to associate multiple records and record types with a call or task. However, the difference between the standard activity panel and Activity Members is Activity Members is limited to logging calls and creating tasks.
Important distinction: Currently, the Activity Members component only works in the Contacts object. Additionally, it does not currently offer email composition or event creation, and report functionality is limited. Because of this, we recommend you only share this with your client if these conditions comply with their workflow.
Adding the Activity Member Component to the Page Layout
1) Go to Contacts
2) Open any contact record
3) Click the Setup cogwheel on the top right corner of the page
4) Select Edit Page from the menu
5) Once in the Lightning Page Editor, verify that you have the Activity Panel included in your current layout. It should appear as indicated in the picture below. if the Activity Panel is missing, please refer to this article on how to customize record page layouts. Click on the Activity Panel, then click Add Tab on the right.
6) This will add an additional tab - click on it to open a picklist, then choose Custom. Now, name your tab. In this example, the tab will be called "Client Activities (Multiple)":
7) Next, click on the newly created tab (Note: this newly created tab may be labeled as "More" if you have multiple tabs or if the label exceeds the typical character limit). You will notice that the component is empty.
8) Navigate to the left panel containing all the standard and custom components. Scroll down to the custom components section. Drag and drop the ActivityMember component onto the space provided under the new tab.
9) Click the Activate button, then decide if you want to make this the default layout for all users in your org.
10) Lastly, click Save, then click Back.
Adding the Activity Member Related Component to the Task Object
Optionally but recommended, add a second component called "ActivityRelatedList" to the Tasks object. The reason for this is to grant better visibility to those records associated with the task or call. The component is also great because it allows you to filter which record types you would like to view in addition to hiding record types as well. The video below will guide you through adding this component as well as using its filter functionality.
Please reach out to Product Support (firstname.lastname@example.org) if you have any questions about this component.