It can be difficult to say whether meetings from distribution lists will appear in FirstAgenda. It behaves differently from the various calendar systems. In some cases, the meetings will not be displayed as no information is sent about who is in the list. For example, the mail "firstname.lastname@example.org" will be considered as a single user, with the initials "team" and not as a complete list containing a number of different participants.
As the meeting creator, if you want to be sure that all participants can see the meeting in FirstAgenda, click the "+" icon or the downward arrow ⌄ when inviting to the meeting in Outlook.
That will make the list expand, and as soon as the list is expanded, all the participants will be able to see the meeting in FirstAgenda.
Read more about distribution lists here: Create contact group or distribution list in Outlook