Setup Muliple Account for Contact | Associate a Contact with Multiple Accounts

Contacts to Multiple Accounts lets your sales reps simply manage the relationships between folks and businesses while not making duplicate records
Enable Contacts to Multiple Accounts, modify page layouts, and customise your connected lists thus your sales reps will simply track relationships between individuals and also the businesses they work with

NOTE:- If you disable Contacts to Multiple Accounts, confine mind that doing thus deletes all indirect account-contact relationships. Associations between a contact and its primary account stay.
 
Setup Muliple Account for Contact 

1) Click on Setup, enter Account Settings in Quick Find, then choose Account Settings.

2) choose permit "Allow users to relate a contact to multiple accounts"

3) Add Contacts related list to the account page layouts that your reps use.

(Above image is used from trailhead module)

4) Add the Related Accounts list to the contact page layouts your reps use.

How to use

Step 1:- Click on Account object and go to "Related Contacts" Related list.

Step 2:- Click on Add Relationship button

Step 3:- Save.


For more detail please check below post :-
1) https://releasenotes.docs.salesforce.com/en-us/summer16/release-notes/rn_sales_shared_contacts.htm

2) https://trailhead.salesforce.com/admin_intro_accounts_contacts/admin_intro_accounts_contacts_relationships

Thanks
Amit Chaudhary