"After months of training, she finally joined the marathon as a professional runner."
In plain English: Joined means when two things are connected or put together to form a single unit.
"The two cities joined forces to build a new bridge."
Usage: Use "joined" to describe the action of connecting or becoming part of a group that happened in the past. It functions as both the simple past tense and the past participle of the verb "join."
Adjective
1
of or relating to two people who are married to each other
"After twenty years, they finally joined as a couple in a simple ceremony at her parents' house."
Derived from Old French joindre, this past participle originates from the Latin verb iungere meaning "to yoke" or "to bind together." It retains its original sense of connecting two things into one.