How to (and should I?) add a tenant to an existing lease

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I have a tenant who is three months into her lease. She's been great so far, pays on time, quiet, no problems. Her boyfriend lives out of town and has spent a few weekends. Again, no problems. I live in the other half of the duplex and haven't noticed an increase in noise or anything else.

He got a job in town starting next month and wants to move in. I have no problem with him moving in.

My question is whether its worth it to add him to the lease. To start, I gave him our typical application and it all looks good. Now, is it necessary to actually add him to the lease? And if so, how do I go about doing that?

I appreciate any input.