If you invite a user to join your team in SigParser but can't join here are a few things to try.

If they didn't get the email...

Have them go to SigParser and sign up for a free account using the email address they were invited with. They'll see the prompt to accept the invite. The email isn't special.

If they aren't being prompted to join the team...

They might have created an account on SigParser prior to the invite being sent. Please have them delete their SigParser account and then signup again. They should then see the invite. Users can only be a member of one SigParser team at a time.