Using Linq instead of multiple foreach loops
The LINQ would be:

var participantList = communities.SelectMany(c =&gt; c.Tournaments
                                                   .SelectMany(t =&gt; t.Rounds
                                                                     .SelectMany(r =&gt; r.Matches)))
                                  .Where(m =&gt; m.Home.ParticipantId.IsNotNull() ||
                                              m.Away.ParticipantId.IsNotNull())
                                  .ToList();


LINQ does not add much imo, if the logic was more complicated the for loops are nicer to debug.

One downside with LINQ for this is that it requires formatting to be readable. If you rename things the formatting needs to be maintained.

With the foreach loops you get formatting for free.

Edit:
As per @RobH's suggestion:

var participantList = communities.SelectMany(c =&gt; c.Tournaments)
                                 .SelectMany(t =&gt; t.Rounds)
                                 .SelectMany(r =&gt; r.Matches)
                                 .Where(m =&gt; m.Home.ParticipantId.IsNotNull() || 
                                             m.Away.ParticipantId.IsNotNull())
                                 .ToList();


Chaining is indeed nicer.