Attaining a part of the congregation is better than leaving it

I attained the opening takbirs with the congregation, but I did not attain a complete rak'ah. Am I considered to have attained the congregation?
If you have someone with you and you want to pray the congregational prayer from the beginning, then yes. If you do not have anyone with you, enter with them even if it is during the tashahhud; because attaining a part of the prayer is better than leaving it all.