跟 as a preposition means "with", and 一起 means "together". This pattern is used to indicate that A and B do something together. Here 跟 can be replaced with 和 without changing the meaning; and the phrase 一 起 is optional.
e.g. 我跟 (or 和) 她去看電影。
I go to see a film with her.
我跟她一起去看電影。
She and I go to see a film together.)
晚上你們跟誰一起去劇場﹖
With whom are you going to the theatre tonight?
A: 晚上你有事兒嗎﹖跟我一起去看京劇﹐好嗎﹖
Do you have anything to do tonight? Come with me to the Beijing
opera!
B: 我不想跟你去﹐我想跟我男朋友一起去。
I don't want to go with you, I want to go with my boyfriend.
N.B. This pattern of A 跟 B (一起) is always placed /before/ the main verb in the sentence. Hence the following sentence, with English word order, is wrong:
我想去跟我男朋友一起。
I want to go together with my boyfriend.