You can join a crew via Rockstar Games Social Club website. Once you register there, you may log in. Then click on CREWS. On the following screen you will be given five options. Click on FIND A CREW. Then you can choose one of the randomly selected crews. You can also find a crew by typing in its name in the field marked with a magnifying glass icon. There are two types of crews - open and closed. You can join open crews freely - just click on JOIN to become a member the group. If you want to get inside a closed crew, you need to request an invitation, then it is up to the crew's leader whether to invite you or not.
You can also join a crew directly from your console. Open Online tab in game menu and choose Crews from the list. Another list will appear on the screen. In Suggested Crews tab you will find randomly selected crews that you may join. If you decide to join one of them, click action button on the group's name and select Join the crew. You can check out an adequate tab to see crews that your friends are in and join them by following the same steps as in joining a suggested group. In Invitations tab you will find invitations to crews sent by other players. You may accept or decline these.
NOTE: You can be a member of up to five groups at the same time, however, only one may be active.