By default when you try and assign a task to multiple users, SharePoint won't let you. this can be rectified in 2 ways.
1. Go to the list in question and then access the list settings, then edit the 'Assigned To' Column, there is the option to 'Allow Multiple Selections', this needs to be set to 'Yes'.
2. At the Site Level: Site Actions > Site Settings > Modify All Site Settings > Galleries - Site Columns. Find the 'Assigned To' column and click on it amend the option to 'Allow Multiple Selections', to be set to 'Yes'.
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