If you know their userprincipalnames you can use the in operator
SigninLogs
| where TimeGenerated > ago(14d)
| where UserPrincipalName in~ ("user1@domain.com", "user2@domain.com", "user3@domain.com")
If you want to use multiple contains, you want the 'or' operator, and would mean a sign on log would need to match all the conditions
SigninLogs
| where TimeGenerated > ago(7d)
| where UserDisplayName contains "Bob Smith" or UserDisplayName contains "Jane Jon" or UserDisplayName contains "Dinesh G"