Join Distribution List in Microsoft 365
In Outlook on the web click the settings gear
Start typing 'Distribution' in the search box, select 'Distribution groups'
Under 'Distribution groups I belong to' click the join button
In the pop-up enter the name (or partial name) of the distribution list in the search box and hit enter, select the list you would like to join and then hit the join button
Click OK in the pop-up after successfully joining the list