'Invite' and 'Invite In'

To "invite (someone) in" means to ask (someone) to come inside of a house or building. To "invite (someone)" means to ask (someone) to attend an event (such as a party) or to do something. Below are some examples of how each phrase is used:

• It was cold outside when they arrived so I invited them in right away. [=I asked them to come inside as soon as they arrived.]
• When they arrived to pick us up for the trip I invited them in for coffee first.
• Be sure to invite Cheryl in when she comes to pick up her son.
• Vampires can't enter your house unless you invite them in.
• I hope Erin and Jess will be at the party. Did you invite them? [=Did you ask them to attend the party?]
• Be sure to invite your cousins to the wedding.
• Let's invite them to dinner tomorrow night.
• I'm going to invite him for coffee soon.
• "What are they doing here?" "I invited them. [=I asked them to be here.]"

I hope this helps.