Hello,
You need to specify what you mean. Join what? What group page? When writing in Questions and Answers forum, try to be as specific as you can with your question, and give as much relevant information as possible - this way other members can quickly and more easily respond and help you. You mentioned an alert, what does this alert say? Nonetheless, I will try to help you as best as I can with your information that you supplied.
In-Game Groups: If you mean in-game groups such as gangs, mafias and business groups, you need to be invited by whoever can invite in that specific group. Please note that you are not allowed to metagame players
(in this case look at the group name under their player name) to find a member of the specific group you are looking to join. You need to find this out via roleplay. If you want to take the easy route, you can simply find their forum profile and message them. Also, most in-game groups have a recruitment page here on the forums, go take a look!
https://www.roleplay.co.uk/forum/267-rebels-gangs/
Whitelisted Factions: There are 3 whitelisted factions, Police, NHS and UNMC. You need to apply here on the forums to be able to join these. I will supply you with links that direct you to the application for each whitelisted faction. But before you apply, you really should read abit on what it means to be in these factions and what is required of you if you were to join. Please note, UNMC is not an apply and join faction, they have to contact you if you are of intrest to them.
If you apply to any of these factions, good luck! I really recommend joining one of them, as it gives some great structured play scenarios and you will get a lot of new roleplay opportunities.
Forum Groups: If what you ment is forum groups, you simply have to go onto that groups forum page, and if they accept people to join, you can press "Join Group". Here you can find the forum groups:
https://www.roleplay.co.uk/groups/?sort=members
I hope that this helped with your question. If you ment none of the 3 "group types" I talked about above, please do specify what you ment. Other than that, have fun and good luck!