Anyone can join a Daily Chess tournament as long as they meet the criteria for the specific tournament (usually related to ratings).
Tournament Directors (TDs) have the ability to change the requirements on a per-tournament basis, but the default criteria is given below:
Must have completed at least five rated games of Daily Chess on Chess.com (required for all tournaments).
Must have no more than a 10% timeout ratio in the last 90 days. (See here for more on that.)