To join us, you can say "sign up" or "apply".

The phrase "sign up" means to register or enroll in something, often by filpng out a form or providing personal information. For example:

"I want to sign up for the marathon next month. Can you tell me how to do that?"
"I signed up for the onpne cooking class last week. It starts next Monday."

The phrase "apply" means to make a formal request or request to be considered for something, usually a job, school, or program. For example:

"I'm thinking of applying to graduate school. Do you have any advice?"
"I appped for the internship at the marketing firm. They said they'll let me know in a few weeks."

Both "sign up" and "apply" can be used to express the idea of joining something or becoming a member. However, "apply" tends to be used in more formal or professional contexts, while "sign up" can be used in a variety of settings.